|Entry into the Land of the Free is still free|
The Foreign Office is warning that thousands of tourists could be turned away at US airports and ports, as a new online entry system comes into effect.
From 12 January, visitors from countries which do not need visas will need to fill in an electronic form at least 72 hours before they travel.
Those who have not registered risk being detained and sent back home.
The Foreign Office fears some people do not know about it and critics say it might put people off visiting the US.
The new online registration scheme replaces the green I-94 forms that people on short term visits to the US had to fill in on the flight and hand to customs on arrival.
America welcomes nearly 60 million tourists a year and about 50 million of those travel without the formality of a visa.
Britain is one of the countries that signed up to the visa waiver programme, but from Monday, new rules apply.
Electronic applications – known as Esta (Electronic System for Travel Authorisation) – have to be approved by the US Department of Homeland Security.