An inquiry was carried out after a manager alleged Royal Wolverhampton NHS Trust was manipulating its death rates.
The Freedom of Information Commissioner has warned the NHS Trust Development Authority failure to respond to a BBC request could be contempt of court.
The authority said it was having difficulty finding all the information.
The independent inquiry begun in March 2014 after manager Sandra Haynes Kirkbright made the allegation.
Led by judge Lucy Scott Moncrieff, it was meant to provide a swift examination into Chief Executive David Loughton's treatment of whistleblowers.
Former chairman Richard Harris, a non-executive director, David Ritchie, and cancer specialist David Ferry -  a consultant who revealed patient safety concerns - are among those known to have given evidence.
Ms Scott Moncrieff confirmed her report was completed last March but it has yet to be published by the authority for legal reasons.
The BBC made an FOI request on 27 October to find out what was holding up the report, asking for all communications between the authority and Department of Health regarding it, as well as between the authority and the trust.
On 5 January, the BBC contacted the FOI commissioner after three attempts to get a response from the authority within the statutory 20 days failed.
It was given a further 10 days to reply, but has yet to do so.
On January 25, a spokesman for the authority said: "We have faced difficulties in locating all of the documents relevant to your request which has contributed to the delay.
"We are still working to retrieve these to ensure that we can share with you as much relevant information as possible."
The commissioner told the authority to provide a "substantive response" to the BBC request within 35 days, warning failure to do so may be dealt with as contempt of court.
The authority said the report's publication was expected shortly and it would then consider the need for any further reviews into the running of the trust.
An NHS regulator must release details about a delayed report into a trust's treatment of whistleblowers or face possible court action.