LONDON - NEW rules went into effect on Monday requiring people travelling to the US under the visa waiver program to register online in advance, instead of filling out paper forms in flight or at the airport.

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

There were no signs of confusion on Monday as the new system was implemented at London Heathrow's sprawling Terminal 5 - departure point for many US-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 US.

Derwood Staeben, US consul general in London, said nearly all applications would be approved in less than 10 seconds. He said travellers 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.' Travellers filling out the online form will be told whether their request is authorised, denied or pending, he said. Those who are marked 'pending' must check back in 72 hours to see if they have been approved, he said.

People whose application is denied must apply to the US Consulate for a visa, he said.

He said the US Congress mandated the change.

The US Homeland Security Department said in a statement that it will take a reasonable approach to travellers who have not obtained an approved travel authorisation, but warned that travelers without authorization do risk being delayed or denied entry to the US.

There is no fee for the service, he said, and the travel authorisation is valid for two years. He warned that operators of some unscrupulous websites are charging fees for this service, which is designed to be free of charge.

People can apply for travel authorisation at any time, and Mr Staeben said about 14,000 applications per day are being received. -- AP