Holidaymakers are being warned not to get caught out by the "passport 10 year rule" which can ruin EU travel plans.
UK travellers used to be able to carry up to nine months from an old passport over onto a new one. But post-Brexit, EU countries will not accept passports issued more than 10 years ago.
Home Office data suggests 32 million people had applied for passports that if granted would be over 10 years old.
Among them is Nathan Barnes who was refused boarding on a flight to France.
Mr Barnes, a 31-year-old paramedic from Norwich, was on his way to Limoges with his fiancé to visit family.
Despite checking in for the flight online, and going through security and passport control, he was stopped at the departure gate.
"My passport had been issued more than 10 years previously," he said. "They were very matter of fact about it, they just said 'sorry you can't board, off you pop'."
"I was gutted, surprised really. We had checked in online and thought it was fine," he added.
Since the UK left the EU, travellers heading to all countries within the zone – and including Iceland, Norway, Lichtenstein and Switzerland, but not Ireland – must have a passport which was issued less than 10 years before their departure date.
It must also be valid for three months after their planned return date.
However, if a UK passport was issued before September 2018, it could in fact be valid for up to 10 years and nine months.
This is because before that the passport office used to add up to nine months from an old passport onto a new one.
Home Office data suggested that between March 2014 and September 2018 there were 32.1 million applications for a UK passport.
'Catches people out'
"The '10-year-rule' is a problem that continues to catch people out", said Rory Boland, the Travel Editor at Which?
"What's really important is that you check your passport validity and expiry date when booking your holiday.
"Not when you go to check-in, and certainly not when you travel to the airport, because that's when people are continually finding that their passport is out of date and it's often then too expensive, and too complicated to save their holiday."
BBC News asked the major airlines and holiday operators for data on the number of passengers who are refused boarding, but those that responded said they did not keep detailed records.
'Holidays lost'
Simon Calder, travel correspondent at the Independent says based on his own research "easily a couple of hundred people a day" are being turned away from their flights.
"Across the year you could easily find that there's over 100,000 people who lose their holidays as a result of these rules," he said.
He has written to the airlines, urging them to do more to make people aware, particularly if someone is checking in and inputting their advanced passenger information, often weeks before their travel date.
Airlines UK told us there was no guarantee that potentially invalid passport details would be picked up in advance, and said airlines do provide reminders and links.
It said it was the responsibility of the person travelling to ensure they have a valid travel document.
The Home Office also said it provided up-to-date information on passport requirements on its website, which it keeps under constant review.
Fees for new passports and renewals are set to go up in April under government plans. A standard online application for an adult is expected to rise from £82.50 to £88.50 from 11 April. A passport for under 16s is currently £53.30 and is set to increase to £57.50.