British travellers could be caught out by new rules which govern the type of passport needed for entry to the US.
Anyone with the wrong format passport would be refused entry under rules which came in on April 1.
Under the Visa Waiver Programme Improvement and Terrorist Travel Prevention Act 2015, anyone visiting America need to have an e-passport.
An e-passport, or biometric passport, is identified by a rectangle on the front cover intercepted with lines and a circle in the middle.
As British passports expire after 10 years, people affected are those who had their passport 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 new 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.
"CBP also sent an email to the applicants whose passports would no longer be valid for travel, as long as a valid email address was on file with their applications."
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 onward travel to another country.
If travellers don't want to get a new passport for their trip, 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.
This visa can be applied for and obtained from the nearest US embassy or consulate, according to advice on the Home Office website.