Britons must have e-passports to visit America or risk being turned away, travellers have been warned.
More than three million Britons could be caught up in the change which became law on April 1.
It is understood the British passports affected are those issued between April and October 2006 - before the introduction of the biometric passport.
The change is stipulated under the Visa Waiver Programme Improvement (VWP) and Terrorist Travel Prevention Act 2015.
Currently 38 countries are 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.
Those from VWP countries without an e-passport who wish 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.