The new program, designed to improve U.S. security, has been voluntary since August but became mandatory yesterday. Travelers are being asked to fill out the forms at least 72 hours in advance of travel.

There were no signs of confusion yesterday as the system was implemented at London Heathrow's Terminal 5 - departure point for many U.S.-bound flights.

"I knew about it because my travel agent told me, so I had already taken care of it online," said Jo English as she checked in for a business trip to Miami.

The rules cover the citizens of 35 countries - from nations in Europe and Asia, to Australia and New Zealand - who don't require a visa to enter the United States.

Derwood Staeben, U.S. consul general in London, said nearly all applications would be approved in fewer than 10 seconds. He said travelers would not be required to give any more information than is already requested on the paper immigration forms, which are being replaced.

"The important change is that we're automating the existing process and requiring it to be done in advance," he said. "The response time is generally about four seconds."

Travelers filling out the online form will be told whether their request is authorized, denied or pending, he said. Those who are marked "pending" must check back in 72 hours to see whether they have been approved, he said. People whose application is denied must apply to the U.S. Consulate for a visa, he said.

He said Congress mandated the change.