LONDON, England (CNN) -- This month the U.S. government has introduced major changes that will affect millions of travelers to the U.S. who do not need a visa.



Instead of filling in the green visa waiver form en route, short-term visitors must register their details online at least three days before they depart.



The measure is designed tighten security and make it harder for terrorists who are citizens of the participating countries to easily obtain entry to the U.S.



The new system, known as the Electronic System for Travel Authorization (ESTA), does not become compulsory until January 12, 2009. But travelers are urged to prepare for the new rules in advance.



ESTA applies to citizens from the 27 Visa Waiver Program (VWP) countries that includes most of western Europe in addition to New Zealand, Japan, Brunei and Australia.



Visitors are recommended to submit applications no later than 72 hours before departure in case further inquiry is necessary. But the U.S. Department of Homeland Security has assured travelers that the system can handle last-minute and emergency requests.



Applications can be made at any time, even if travelers have no specific travel plans. And if itineraries change, information can be easily updated on the ESTA Web site.



Once travelers are authorized, they can travel for up to two years or until their passport expires, whichever comes first.



From mid-January, travelers who have not received approval may be denied boarding, delayed processing, or denied admission at a U.S. port of entry.



Passengers must submit the same information that is currently required in the I-94 immigration form. This includes biographical data, travel information as well as questions regarding communicable diseases, arrests and convictions.



Registration is possible through the U.S. government ESTA Web site. In most cases, eligibility for travel will be approved immediately. Applicants who receive an "Authorization Pending" response will need to check the Web site for updates. Applicants whose ESTA applications are denied will be referred to Travel.State.Gov for information on how to apply for a visa.



ESTA does not change the rules for citizens from countries that require visas. But the U.S. is due to extend the visa waiver program and has signed agreements with eight countries including the Czech Republic, Hungary and South Korea.



Travelers are advised that ESTA does not guarantee entry into the United States. The final decision rests with the immigration official at the port of entry.
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question: How long could the process take?, answer: most cases, eligibility for travel will be approved immediately. | question: What do the new rulres require visa-free visitors to the U.S. to do?, answer: register | question: How long can travelers travel once they are authorized?, answer: up to two years or until their passport expires, | question: What system debuted this month?, answer: for Travel Authorization | question: What do the new rules require visa-free visitors to do?, answer: must register their details online at least three days before they depart. | question: How many hours can the process take?, answer: 72 | question: When does the system become mandatory?, answer: January 12, 2009. | question: When will the system become compulsory?, answer: January 12, 2009. | question: What do new rules require?, answer: short-term visitors must register | question: The process could take how many hours?, answer: 72 | question: Is it possible to apply at the last minute?, answer: the system can handle last-minute and emergency requests.