Applicants must give their passport number and other personal details
New entry rules for the US will be implemented "in a reasonable manner," an embassy official has told the BBC.
Earlier, the Foreign Office warned that "thousands of tourists" could be turned away at US customs if they do not fill in an online form before travelling.
But Consul General Derwood Staeben said that was "alarmist" and the new system aimed "to encourage travel to the US".
"There will not be chaos at the border," he said. The rules came into force on Monday.
The new online registration scheme will gradually replace the green I-94 forms that people on short term visits to the US have to fill in on the flight and hand to immigration on arrival.
Known as Esta (Electronic System for Travel Authorisation), it applies to all visitors from countries which do not need visas - believed to be about 50 million people each year.
No time limit
On Saturday, the Foreign Office said "thousands of Britons could potentially be turned away by immigration at US airports if they fail to register".
"British travellers who have not registered before their trip are likely to be detained and sent home.
"This may affect a wide range of British nationals and the Foreign Office is particularly concerned that people who may not have heard of ESTA and booked their trip before enforcement of the new rule may be caught out."
The Foreign Office had said the forms must be completed at least 72 hours before departure, but Mr Staeben said that was incorrect.
"You can fill it in from an internet cafe in the airport if you want to, although obviously we do advise people to do so as far ahead of their journey as possible," he said.
Mr Staeben said there was no limit to how far in advance the form could be completed and insisted that rumours of thousands of people being turned away were incorrect.
"We are implementing this in a reasonable way," he said.
"People who don't do it can expect to be delayed, denied boarding or turned away - that is technically true - but we're working with the travelling public on this.
"We will take a reasonable approach. We want to encourage travel to the US."
Once completed, electronic applications have to be approved by the US Department of Homeland Security.
The US Embassy in London said so far 99.6% of the applications had been approved - most within four seconds.
Once an application is approved, it will be valid for all visits to the US for a two-year period, or until the applicant's passport expires.
Earlier, Michael Restovich, from the US Department of Homeland Security, said: "We want to keep the bad people out."
Abta - the Travel Association - believes the new system will help speed up the immigration process.
It pointed out that Esta is free, but warned that searches for Esta online brought up numerous websites offering to process the application in return for personal details and a fee.
Critics say it will be an inconvenience for business travellers and could prove a disincentive to people who like to travel spontaneously and book last-minute weekend breaks to US cities.