A tightening of rules at the start of April in the wake of another shooting means anyone visiting the country must have a bio-metric epassport.
However, Brits who were issued passports between April and October 2006 are now being refused entry as their documents no longer qualify.
Instead, they must apply for a visa from the US embassy or consulate, a process that can take up to 72 hours.
With six million passports issued in the UK each year, it is possible that as many as three million people could find themselves ineligible to visit the country.
