New rules requiring Britons to register online before travelling to America could lead to thousands of tourists being turned away at US airports.
Those who have not logged themselves are likely to be detained and sent home, the Foreign Office (FO) has warned.
From Monday, Britons travelling to the US on the visa waiver scheme will need to fill in an electronic travel authorisation form 72 hours before their trip.
The information travellers provide is the same as they are required to produce on the forms they fill out on planes flying to the US.
Travellers do not have to specify exactly when they are travelling and the electronic authorisation, which is replacing the on-plane forms, lasts for two years.
Electronic applications – known as Esta (Electronic System for Travel Authorisation) – have to be approved by the US Department of Homeland Security.
The US Embassy in London said that so far 99.6% of those registering have been approved – most within four seconds.
The FO said: “From January 12, thousands of Britons could potentially be turned away by immigration at US airports if they fail to register on Esta 72 hours prior to their departure.
“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.”
Andrew Spice, of Post Office Travel Services, said: “Holidaymakers could get caught out if they fail to apply in time and it may prove a disincentive to people who like to travel spontaneously and book last-minute weekend breaks.
“Problems may also occur if UK tourists travel to the USA via another country – like Canada or the Caribbean Islands – and don’t realise that they will need the Esta to gain entry.”
US ambassador to the UK Robert Tuttle said: “Esta is one way that we are trying to make travel to the US even easier.
“I urge everyone thinking about a trip to the US to complete an Esta application to ensure a smooth trip.”
Jim Forster, British Airways’ government and industry affairs manager, said: “The website is quick, free and easy to use and the Esta remains valid for two years.”
