Travellers with older-style passports have been warned they need to upgrade to a biometric one to be allowed into the US.
New rules that came into effect on April 1 mean only people with a biometric e-passport will be allowed access to the US.
Families with holiday plans, as well as those on business trips, could be affected, with the security clampdown being introduced following a wave of terrorist attacks in recent months.
Anyone who has been issued with a passport since e-passports were introduced in October 2006 will have th latest biometric version.
Since passports only last 10 years, only those with passports issued between May and October 2006 will be affected if they have travel plans to the US.
Brits travelling to the US do not need a visa but must apply for an ESTA –Electronic System for Travel Authorisation.
The normal requirement that a passport is valid for six months beyond the holder’s stay is waived.
With 3.8 million British travellers visiting the US every year, there are thought to be thousands of people still with older-style passports.