The US government is to start charging UK travellers $14 (£9) to apply for permission to enter the country.
The compulsory Electronic System for Travel Authorisation (Esta) is free at present, but from 9 September visitors to the US will have to pay for it.
It lasts for two years; people who already have a valid form will not have to pay until their current one expires.
The scheme was introduced in January 2009 but was made mandatory from January 2010.
It applies to all 35 countries with US "visa waiver" status.
The Electronic System for Travel Authorisation form, which takes up to 72 hours to be approved, gives air passengers prior approval for entry to the US.
It replaced the green I-94 card which passengers have previously filled in on their flight into the US.
End Quote Martin Lewis MoneySavingExpert.com
Most people who fill in the form should receive approval from the US Department of Homeland Security within a few minutes.
But British Airways and American Airlines are advising travellers to apply at least 72 hours in advance.
Both airlines have said people without valid Esta forms or visas will not be allowed to board flights to the US.
The fee has been introduced to fund a programme which aims to promote tourism in the US and attract foreign spending.
Martin Lewis, creator of MoneySavingExpert.com, has urged people even considering a trip to the US to apply for a form now to beat the charge.
He said: "The security authorisation lasts for two years, so do it now and it'll be free if you travel in that time. Yet leave it for a month and you'll have to pay for it.
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