LONDON, England (CNN) -- From today, travelers visiting the United States under the Visa Waiver Program (VWP) risk being detained at airports and sent home if they don't comply with new U.S. immigration rules.
Thousands of travelers risk being detained and sent home from U.S. airports and ports.
The introduction of the Electronic System for Travel Authorization (ESTA) means visitors from 27 VWP countries -- including most of Western Europe, New Zealand, Japan and Australia -- must now register their details online at least three days before departure.
ESTA -- which came into effect today -- replaces the written green I-94 form and allows travelers under the VWP to enter the U.S. without a visa and stay for up to 90 days.
The measure is designed to tighten security and make it harder for terrorists who are citizens of the participating countries to easily obtain entry to the U.S.
Critics fear the new rule will be an inconvenience for business travelers and the British Foreign Office is concerned that travelers who have not heard about the new rules may be caught out.
A British Foreign Office report on travel trends for 2009 predicted that 13 percent of British travelers are more likely to visit the States now Barack Obama has been elected. "The consequences of not registering through ESTA could therefore be far reaching," says the Foreign Office.
The U.S. Department of Homeland Security has assured travelers that the system can handle last-minute and emergency requests.
ESTA has been operating on a voluntary basis since 1 August 2008 and is compulsory from 12 January 2009.
Applications can be made at any time, even if travelers have no specific travel plans. 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.
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.
That no longer includes travelers from the Czech Republic, Estonia, Hungary, Latvia, Lithuania, the Republic of Korea, and the Slovak Republic; they joined the VWP in November, 2008. Malta became a member the following month.
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.