Under the Electronic System for Travel Authorization, visitors from 27 countries in the Visa Waiver Program must register their information online at least three days before departure, CNN reported. The measure is designed to tighten security and make it harder for terrorists who are citizens of the participating countries to enter the United States easily.

ESTA replaces the written green I-94 form and permits travelers under the VWP -- countries in most of western Europe, New Zealand, Japan and Australia -- to enter the United States without a visa and stay up to 90 days.

Critics said they were concerned the new rule would inconvenience business travelers while the British Foreign Office expressed concern that travelers who haven't heard about the program may be caught in the middle.

The U.S. Department of Homeland Security is assuring travelers the system can handle last-minute and emergency requests, and that the program has been operating on a voluntary basis since August.

Applications may be made at any time, even if travelers don't have specific plans. If itineraries change, information can be updated on the ESTA Web site, Homeland officials said.