British travellers are being warned to check their passports or risk being turned away from the United States.
As of April 1 this year, under the Visa Waiver Programme Improvement and Terrorist Travel Prevention Act 2015, those visiting America need to have an e-passport.
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It is understood the British passports affected are those issued between April and October 2006 - before the introduction of the biometric passport.
And with six million of the maroon coloured travel documents handed out each year, more than three million Britons could be caught up in the change.
The rules, introduced by the US government, state that visitors travelling to the country from a Visa Waiver Programme (VWP) country need a passport with the embedded electronic chip.
A spokeswoman for US Customs and Border Protection (CBP) said after the law passed it began posting on its own and the Electronic System for Travel Authorisation (Esta) websites about the new rule.
She added: "CBP encourages travellers to check their passports and their current Esta status to make sure they are in compliance with the requirement.
There are currently 38 countries signed up to the VWP - including the UK. The scheme allows passport holders from those nations to visit the US for a period of up to 90 days.
Visits that can be made include business, tourism and travel to another country.
The CBP spokeswoman said those from VWP countries without an e-passport and who wished to travel to the United States could obtain a non-immigrant visa.
According to advice on the Home Office website, this visa can be applied for and obtained from the nearest US embassy or consulate.