opinion: J. JOSEPH SMITH, Circuit Judge:
  This appeal raises interesting and difficult questions concerning the balance to be struck between prisoners’ sixth and fourteenth amendment rights and the need to allow prison officials discretion to exclude communications they feel will endanger their institutions. The United States District Court for the Southern District of New York, Charles Tenney, Judge, granted a preliminary injunction to plaintiffs, prisoner-members of the Prisoners’ Labor Union at Green Haven (“the union”) holding that they must be allowed to receive a letter from their attorneys containing legal advice about the formation of the union and efforts to have it officially certified, and all letters from the Legal Aid Society, in accordance with the provisions of Administrative Bulletin No. 20 (January 31, 1972) of the Department of Correctional Services. The order was not given effect, as this court granted a stay and expedited the appeal of the Commissioner of Corrections and the Superintendent of Green Haven, who claim that the court below erred in his estimate of the effect the letters would have and abused his discretion in granting preliminary relief and allowing the letters to enter the allegedly tense prison. We find error only in the extent of relief granted and modify and affirm the grant of preliminary injunction.
  The case presents three basic issues: (1) whether the court was incorrect in finding, on constitutional grounds, that the officials were unjustified in withholding the letters; (2) whether it was proper to require obedience to Administrative Regulation No. 20, which sets a standard for censorship of legal mail that is more lenient than constitutionally required; and (3) whether the conditions which justify issuance of a preliminary injunction exist. We are not faced on this appeal with the question of the constitutionality or legality of unions or other organizations of prisoners, but only with the right of prisoners to receive communications from counsel whose advice has been sought on that question. We do not therefore intimate any views as to the legality, desirability, dangers or possible benefits of any type of prisoner collective bargaining on prison working conditions or of any other organized representation of prisoners.
  The inmates at Green Haven, a maximum security institution at Stormville, New York, holding 1900-2000 men, began during the summer of 1971 to organize a labor union to act on their behalf in connection with conditions of labor in the prison. They contacted the Legal Aid Society’s Prisoners’ Rights Project for assistance in this endeavor. The Project was told, when it inquired of the Commissioner, that prisoners are not in an employee-employer relationship with the Department of Corrections and that the Department would not recognize any inmate labor organization because “it would be contrary to the best interests of the Department and of the general welfare of the prison population.” In December, 1971, the Project received the signatures of approximately 800 inmates requesting the attorneys to draft a union constitution and represent them in union-related matters. The constitution was sent to the inmates; though it may have been signed and become effective, by its own terms, the union has thus far taken no action. Legal Aid also provided authorization forms to sign up new union members.
  The communication which is the subject of this suit was sent by Legal Aid during the week of February 7, 1972 to all those inmates who had signed up as members of the union but to no one else. In the packet sent to each inmate the main document was a seven-page letter detailing legal steps being taken on behalf of the union and giving legal advice on a number of union matters. A copy of the letter to the Commissioner requesting recognition of the union, the union constitution, and press releases issued February 7 announcing the formation of the union and commitments of support for it by prominent individuals, were included. On February 9, the Commissioner told Legal Aid that he would deny recognition to the union; a week later he informed the Society that he had not and would not deliver the February 7 letters to the 980 addressees. Apparently, fifteen of the letters had arrived unsealed and were examined by prison officials; on the basis of this scrutiny the authorities had decided to withhold the letters. The next day, the 18th, Legal Aid filed a petition with the Public Employees Relations Board requesting certification of the union as collective bargaining agent for the prisoners. The present complaint was filed on February 23; the action was brought under 42 U.S.C. § 1983 and its jurisdictional complement 28 U.S.C. § 1343, and plaintiffs requested a preliminary injunction ordering the defendants to deliver the communications from their attorneys.
  Affidavits were submitted to the court by both sides; neither requested an evidentiary hearing. The defendants argued that their action was justified because the formation of a union was against prison policy and would jeopardize their control of the institution. They interpreted the letter as a declaration that the union was “operational” and as an incitement to “concerted activity” that would present a clear and present danger of disruption to the institution. They anticipated that if the letters were delivered, and they were then obliged to burst the balloon of rising expectations by proscribing union activity, they would subject themselves to danger.
  The court found that the letter was a communication of legal advice, not a call to illegal action, and that its optimism about the formation of a union was carefully hedged with cautionary instructions to obey all prison rules in the interim, which might be lengthy, before the union was certified and could negotiate with prison officials. The court held that the letter came within the “legal mail” classification of Sostre v. McGinnis, 442 F.2d 178 (2d Cir. 1971), cert. denied, sub nom. Oswald, Correction Commissioner et al. v. Sostre, 405 U.S. 978, 92 S.Ct. 1190, 31 L.Ed.2d 254 (1972), and neither violated the standards applicable to such mail nor presented a clear and present danger to the security of the institution. The court felt that the resort to legal methods of challenging the prison’s refusal to countenance unions and of improving the prisoners’ status was a constructive and rehabilitative rather than a dangerous move. The withholding also violated the Commissioner’s own Administrative Regulation No. 20, and the court ordered that rule followed in the future. He held the class of all inmates proper under Rule 23(b) (2). He was not entirely clear on the issues of irreparable injury and balance of hardships. The only issues for this court are whether the court made clearly erroneous findings of fact or abused his discretion in ordering the warden to deliver the letters from Legal Aid and to abide by his mail regulation in the future. The court’s easy characterization of the Legal Aid letter as pure legal advice may be open to question, as it was somewhat more ambiguous than that, but we hold that he was quite correct in his assessment of the appropriate legal standard and his application of it to the facts of the case.
  The main letter from the Prisoners’ Rights Project attorneys is addressed to “Dear Union Member” and begins by remarking on the public announcement of that day and by noting the possibility of affiliation with already existing unions. The letter goes on: “Now that your union is ready to function we have been requested to provide further legal advice as to the role it can play, how it will operate, and how to deal with problems that may arise.” The letter continues with a recitation of efforts to induce the Commissioner to recognize the union as the collective bargaining agent for the inmates; if he denies it recognition, the attorneys state that they will file a petition with the Public Employees Relations Board (PERB). The letter in general and the above sentences in particular had a positive tone about the probable outcome of the PERB proceeding and the chance that the union would be functional in the relatively near future that undoubtedly seemed unwarranted and threatening to the state. The letter advised inmates that it was crucial that they obey all institutional rules during the certification process before PERB and, if necessary, in the courts.
  The letter goes on to describe the issues the union would treat were it certified ; certain of these paragraphs have a note, once again, of confident assumption that the recognition of the union is an event whose occurrence is quite certain rather than merely marginally possible. Most of the sentences, however, use the verb “would” rather than “will,” thus keeping the tone conditional. The letter mentions the fact that the prison officials have not objected to solicitation of union members over the preceding months and presents the Project’s view on why a union would serve the interests of the inmates more effectively than a liaison committee serving at the discretion of the administration. The letter ends with a warning that should the administration resist the union, the process of obtaining recognition will be long and arduous; the belief expressed is that the union “will eventually succeed” rather than that it will begin negotiations tomorrow.
  The constitution sets forth as union goals the advancement of the economic, political, social and cultural interests of the prisoners, the adoption of laws increasing the welfare of prisoners, and the equalization of the rights of prison labor and free labor by expansion and recognition of the former. There are clauses on dues, meetings, and other routine matters. The press releases raise no problems.
  Before discussing the specific factors operative in this situation, we review the constitutional context in which the claims of the plaintiffs arise. As convicted prisoners, they are denied the full panoply of constitutional rights which citizens normally enjoy. But among the basic rights which they do retain in prison are certain of the first amendment freedoms and the sixth and fourteenth amendment right of access to the courts. It is the latter right which is principally involved here, although there are first amendment overtones, recognized by the lower court, in this situation. The right of a prisoner to access to the courts was first articulated by the Supreme Court in Ex parte Hull, 312 U.S. 546, 61 S.Ct. 640, 85 L.Ed. 1034 (1941). Since then, the boundaries of the right has been further delineated; a necessary concomitant to the right of access is the right to assistance of counsel. See, e. g., McMann v. Richardson, 397 U.S. 759, 771 n. 14, 90 S.Ct. 1441, 25 L.Ed.2d 763 (1970); Johnson v. Avery, 393 U.S. 483, 89 S.Ct. 747, 21 L.Ed. 2d 718 (1969); Gilmore v. Lynch, 319 F.Supp. 105 (N.D.Cal.), aff’d sub nom. Younger v. Gilmore, 404 U.S. 15, 92 S. Ct. 250, 30 L.Ed.2d 142 (1971). In turn, the provision of effective assistance requires the opportunity for confidential communication between attorney and client on pending litigation and related legal issues. Coleman v. Peyton, 362 F.2d 905 (4th Cir.), cert. denied, 385 U.S. 905, 87 S.Ct. 216, 17 L.Ed.2d 135 (1966); McCloskey v. Maryland, 337 F.2d 72 (4th Cir. 1966); Carothers v. Follette, 314 F.Supp. 1014 (S.D.N.Y. 1970); Fulwood v. Clemmer, 206 F. Supp. 370 (D.D.C.1962). This court and others have recognized the primary importance of mail in this attorney-client relationship and have granted it protection from interference by prison staff in all but extraordinary circumstances. Sostre v. McGinnis, supra; Nolan v. Scafati, 430 F.2d 548 (1st Cir. 1970); McDonough v. Director of Patuxent, 429 F.2d 1189 (4th Cir. 1970); Smith v. Robbins, 328 F.Supp. 162 (D.Me.1971), aff’d 454 F.2d 696 (1st Cir. 1972); Marsh v. Moore, supra; Palmigiano v. Travisono, 317 F.Supp. 776 (D.R.I. 1970). See Corby v. Conboy, 457 F.2d 251 (2d Cir. 1972); Wright v. McMann, 460 F.2d 126 (2d Cir. 1972). Cf. Coplon v. United States, 89 U.S.App.D.C. 103, 191 F.2d 749 (1951), cert. denied, 342 U.S. 926, 72 S.Ct. 363, 96 L.Ed. 690 (1952).
  The standard which applies to attorney-client mail in this circuit was formulated in Sostre v. McGinnis, supra; in distinction to other correspondence, which may be read and censored for material which inhibits or threatens rehabilitation, security, or other professed goals of incarceration, mail to or from attorneys is rarely proscribed:
  The generous scope of discretion accorded prison authorities also heightens the importance of permitting free and uninhibited access by prisoners to both administrative and judicial forums for the purpose of seeking redress of grievances against state officers. The importance of these rights of access suggests the need for guidelines both generous and specific enough to afford protection against the reality or the chilling threat of administrative infringement. Thus, we do not believe it would unnecessarily hamper prison administration to forbid prison authorities to delete material from, withhold or refuse to mail a communication between an inmate and his attorney (citations omitted) or any court . . . unless it can be demonstrated that a prisoner has clearly abused his rights of access. [I]f a communication is properly intended to advance a prisoner’s effort to secure redress for alleged abuses, no interest would justify deleting material thought by prison authorities to be irrelevant to the prisoner’s complaint. The chance that an official will improperly substitute his judgment for that of the correspondent then preponderates. For similar reasons, prison officials may not withhold . . . material from otherwise protected communications merely because they believe the allegations to be repetitious, false, or malicious. [442 F.2d at 200-201]
  The state argues that under any of the relevant tests the letter was properly excluded from the prison. Citing Roberts v. Pepersack, 256 F.Supp. 415 (D. Md.1966), which held that it was permissible to punish an inmate who had called for a demonstration against prison conditions, and McCloskey v. Maryland, 337 F.2d 72 (4th Cir. 1964), in which similar restriction was held permissible even though the advocacy was not a call to action but merely a dissemination of political or social views, the defendants urge that this is an analogous situation in which the first and sixth amendment rights of the inmates may be curtailed in order to prevent an incitement to concerted effort to evade institutional rules. Nor, defendants claim, can this threatening behavior hide behind a purported attorney-client relationship when in essence it is not bona fide legal advice. See Sostre v. McGinnis, 442 F.2d at 200; McCloskey v. Maryland, 337 F.2d at 74.
  The answer to the question of which category the letter from Legal Aid falls into — whether it urged illegal concerted action, was merely correspondence by interested persons, or was communication to clients giving legal advice on actions intended to challenge and improve the conditions of confinement-— seems to us reasonably clear, although because prison unions are very new we find no cases directly on point. Most of the cases deal with representation in criminal cases, on habeas corpus petitions, or in civil rights actions, under 42 U.S.C. § 1983. The attorneys here might be said to be taking action analogous, though on a larger scale, to a section 1983 action, trying to establish a right, possibly derived from the first and thirteenth amendments, for the prisoners to associate and try to lessen the harshness of prison work conditions and contribute to the success of training and rehabilitation programs. Nor does the fact that the union was declared “operational” remove this letter from the realm of advice, for there is a valid and significant distinction between the existence of the union and its certification as an operating bargaining agent. To apply to PERB, the union had to be in existence, and the letter made it clear that the group could not deal with the officials yet, that the certification was a potentially lengthy legal proceeding, and that thinking about the issues the union might eventually raise (while obeying all prison rules) was the only concerted action inmates ought to take at present. They were not urged to change their work habits or to present any demands to the administration. A finding that this is not propaganda or incitement cannot be termed clearly erroneous.
  Under the test for attorney-client mail, the state must show clearly an abuse of access in order to justify restriction. Defendants claim that such an abuse exists here, because the letter advocated an “unlawful scheme,” one instance mentioned by the Sostre court in which some restriction would be permissible. The contention that an application for recognition of the union and communication with one’s clients preparatory to such application are components of an unlawful scheme seems a misuse of that term. The lawyers were telling the prisoners to utilize lawful, not unlawful channels for the presentation of grievances and were guiding a challenge to a prison rule through orderly procedures. It is difficult to discern in what other fashion the prison would prefer to have the rule examined; it is the only peaceful method by which it can be reviewed by someone other than the Commissioner or his deputy, who are naturally interested in quelling any inmate activity which may arrogate to inmates themselves some decision-making power about the conditions of prison life.
  Legal Aid points out, as the district court found, the inconsistency between the officials’ tolerance of the solicitation and mailing of authorization forms and the intolerance of a legal opinion which they undoubtedly never expected and with which they vehemently disagree. Given the fact that they allowed the solicitation which raised inmate hopes, their argument about the danger that will result from the disillusionment they will have to cope with on the heels of the Legal Aid letter seems highly disingenuous. They can hardly claim that to allow the prisoners to read a letter will be more disruptive than to deny them all word of the result of their organizational efforts of several months. In fact, the state conceded this point at oral argument; counsel stated that her clients did not object to a letter advising the inmates of the applicable law and recent events, but that the objection to the present letter was that it assumed the union was “operational.” This objection has been disposed of above.
  Despite the fact that the letter was not within that category of legal mail which constitutes an abuse of access, defendants claim that the existence of a clear and present danger to the security of the institution justifies the exclusion. When such a danger exists, it has been held permissible to restrict even those most basic and “preferred” freedoms of individuals. A standard for such danger in a prison context has been set forth in Fortune Society v. McGinnis, supra n. 3, with which we agree: that in order to justify official interference, the state must show “a compelling state interest centering about prison security, or a clear and present danger of a breach of prison security, * * * or some substantial interference with orderly institutional administration.” 319 F.Supp. at 904.
  The Commissioner, in an affidavit by an assistant attorney general, claims that the distribution of the letter would impair the orderly administration of the institution by creating an alternate source of authority to challenge the officials. However, the union’s legality is being adjudicated in another proceeding, and the introduction of the letters will not lead irresistibly to that result before the conclusion of that other procedure; it merely keeps the parties to that proceeding informed of its progress and of issues on which they may have to advise counsel. He emphasized his fear of reprisals when, after delivery of the letters, he is obliged to inform the inmates that the union can never exist. As the very issue of its right to negotiate is being litigated in another forum, he need not and cannot honestly tell the inmates that it “can never” exist, but merely give his opinion on the probable outcome of that proceeding. And defendants’ fears that their own response to the letters will in turn cause a disturbance cannot justify a refusal to deliver them. Finally, the Superintendent states that he expects fights between pro and anti-union inmates.
  These conclusory predictions are based on the fact that there are approximately ten fistfights a week at the institution (which houses 1900 inmates) and that there have been threats. against several guards over a period of months and a few unfounded rumors of “trouble.” But during this past fall of anxiety and disruption at prisons in New York and elsewhere, Green Haven was quiet; and we conclude that no factual basis is shown for the dire predictions of the defendants. Compare the situations in Lee v. Washington, 390 U.S. 333, 88 S. Ct.994, 19 L.Ed.2d 1212 (1968); Rhem v. McGrath, 326 F.Supp. 681 (S.D.N.Y. 1971) ; Long v. Parker, 390 F.2d 816, 822 (3d Cir. 1968). See Davis v. Lindsay, 321 F.Supp. 1134 (S.D.N.Y.1970).
  We conclude that the court was correct in requiring the delivery of the letters and enclosures. The judgment, however, goes further and requires adherence to the present departmental regulation on attorney mail. This regulation goes beyond the requirements of Sostre in requiring that attorney mail be opened in the presence of the inmate, presumably to insure that it is checked only for contraband and is not read for content. This would inhibit the chilling effect of any procedure under which the inmate must trust prison staff not to read an opened letter. While we feel that this policy is desirable, (and has been required in the First Circuit, see Smith v. Robbins, 454 F.2d 696 (1972) ) and urge the Commissioner to pursue it, we think it unnecessary to go so far and to freeze the regulations on this matter on this appeal on the narrow issue of the present Legal Aid letters. The issue here is confined to mail from that source, in accordance with the principles set out herein. The injunction may be modified to require only the delivery of the letters and enclosures described above, and other mail from Legal Aid to the prisoners concerning their union claims and other legal problems.
  . The rule on mail from attorneys is as follows: “You may correspond with any attorney on legal matters or regarding your legal rights. Your incoming and outgoing letters and enclosures will be opened and examined in your presence to insure the absence of contraband. The contraband will be censored.”
  . In Marsh v. Moore, 325 F.Supp. 392 (D.Mass.1971), cited by the court below, the withholding of mail from an attorney to his client concerning a pending legal matter was held to be irreparable injury in the preparation and prosecution of the case, and to warrant preliminary relief.
  . On the first amendment rights of prisoners, see Barnett v. Rodgers, 133 U.S. App.D.C. 296, 410 F.2d 995 (1969); Nolan v. Fitzpatrick, 451 F.2d 545 (1st Cir. 1971); Seale v. Manson, 326 F. Supp. 1375 (D.Conn.1971); Payne v. Whitmore, 325 F.Supp. 1191 (D.Cal. 1971); Fortune Society v. McGinnis, 319 F.Supp. 901 (S.D.N.Y.1970); Sobell v. Reed, 327 F.Supp. 1294 (S.D.N.Y. 1971). See generally.Note: Prison Mail Censorship and the First Amendment, 81 Yale L.J. 87 (1971).
  . As the court said in Nolan v. Scafati, 430 F.2d 548 (1st Cir. 1970), “that prison inmates do not have all the constitutional rights of citizens in society— and may hold some constitutional rights in diluted form — does not permit prison officials to frustrate vindication of those rights which are enjoyed by inmates or to be the sole judges' — by refusal to mail letters to counsel — to determine which letters assert constitutional rights.” 430 F.2d at 551.
  . If the documents do not constitute a call to illegal concerted action against the institution, then the appellants claim that they are merely correspondence between members of the public and the inmates that may be censored, as “many kinds of controls on the correspondence of the inmate” are constitutionally permissible. Sostre, supra, 442 F.2d at 199. Limits on those to whom he can write and on what he can say and censorship to remove offensive material are permissible. See Administrative Regulation No. 20, paragraph 12.
  We think, however, the letter and documents cannot properly be termed merely correspondence sent to the prisoners by interested members of the public. The Legal Aid Society had no involvement in prisoner unions before they received the letter from the prisoners in Green Haven asking for advice on the formation of such a group. The letter specifically gave a legal opinion on how to form a union, its legal status, possible powers, etc. This appears to us bona fide legal advice despite the somewhat over-enthusiastic language in some parts of the letter. That language, given the clearly expressed lack of intent to incite any violence against the institution, does not change the legal nature of the letter.
  . The rule in Sostre and most other cases is not limited to certain forms of legal action but extends to communication on any legal questions and issues intended to obtain redress for alleged unconstitutional rules or actions.
  . Nolan v. Fitzpatrick, 326 F.Supp. 209, 214-217 (D.Mass.1971). Cf. Edwards v. South Carolina, 372 U.S. 229, 231, 83 S.Ct. 680, 9 L.Ed.2d 697 (1963); Terminiello v. Chicago, 337 U.S. 1, 5, 69 S.Ct. 894, 93 L.Ed. 1131 (1949). Of course, the likelihood of the communications themselves causing breach of security or of prison discipline or substantial interference with administration is to be measured in the light of the situation within the institution, rather than the conditions outside portrayed in Edwards and Terminiello.