Millions of Britons with valid passports could be unable to travel to some European countries in the event of a no-deal Brexit.
Consumer group Which? warned that up to 3.5 million people could be blocked from destinations including Spain and France.
The Government had said some passports with as much as 15 months of validity left could be affected by a no-deal outcome.
Some would not be accepted for travel in the Schengen Area, which comprises of European states with a common visa policy, it warned.
Until recently, British passports renewed early could be valid for up to 10 years and nine months.
The extra nine months will no longer be accepted within the Schengen zone if a deal is not reached.
All travellers will need at least six months remaining on their passports to enter most of the EU.
However this would be measured at nine-and-a-half years from when it was issued.
Which? said travel companies and the Government must “urgently” up efforts to ensure accurate information is given.
Its travel editor, Rory Boland, said: "People will be shocked to hear that they could be turned away at airports and prevented from going on holiday - even when their passport is valid for another 15 months.
"As this potentially affects the travel arrangements of millions of people, the Government and travel companies must urgently up their efforts to ensure holidaymakers have clear and accurate information - so that no-one is left stranded at the departure gate."
An amended Brexit deal is set for a vote on Tuesday.
It comes after Theresa May suffered a humiliating defeat earlier this month when around a third of her MPs rebelled.
The Prime Minister is facing calls to extend Article 50, with the March 29 deadline for leaving the EU looming.
Additional reporting by PA
