London (CNN) — UK consumer group Which? has warned that millions of Britons could see holiday plans disrupted if they don't renew their passport by Friday.
Which? claims that 3.5 million people could be prevented from entering 26 countries in the Schengen zone -- including France, Italy and Spain -- in the event of a no-deal Brexit.
UK passport holders were previously told that they would require six months validity on their passport on the day of travel in order to enter Schengen countries after Brexit.
However a legal loophole means that those with up to 15 months' validity could be forced to stay at home if Britain crashes out of the EU without a deal.
Britain's Prime Minister Theresa May is still struggling to finalize a Brexit deal.
BEN STANSALL/AFP/Getty Images
The deadline to reach a deal is March 29, three weeks from Friday.
A new UK passport takes three weeks to arrive using the standard service, leading to claims of a Friday "deadline," and Which? warns that the passport office could be busier than normal as more people realize they need a new document in order to travel.
However expedited services are available for an extra fee, and there is still time for the UK and the EU to reach a deal and end the threat of travel disruption.
Worries over passport validity stem from UK rules meaning that those who renewed their passport before it expired could have up to nine months of the remaining time added to their new document.
But the government recently announced that this added time would not be recognized, potentially leaving those with up to 15 months' validity remaining shut out of popular destinations in Europe.
"Millions of people could find their holiday plans disrupted or worse, yet find themselves stranded at the departure gate and prevented from traveling altogether if they fail to renew their passport before the cut off date," said Rory Boland, Which? Travel Editor, in a statement.
"Anyone who thinks they might be affected should look to renew their passport today or if you already have travel plans in the near future consider using the fast track passport renewal service."
Which? said travelers should check their passport status, and warned that anyone refused travel due to an invalid passport will not be entitled to rebooking, compensation or a refund.
With Brexit fast approaching, travelers are facing huge uncertainty about what will happen on March 29.
And the UK government has acknowledged that people should check their passport status.
"If there is no deal, new rules would apply when traveling on a British passport to many European countries and some people may need to renew their passport earlier than planned," said a Home Office spokesperson.
"If your passport does not meet the new rules, you should apply for a new passport before traveling to the countries affected. You can use the government's online passport checker to check a passport for travel to Europe. We recommend that you renew your passport in good time ahead of your travel date."
Questions over passport validity are part of wider worries over freedom of movement after Brexit.
Assurances have been made by the European Commission that UK travelers will not require a visa to visit the EU, at least initially.
However, the Commission has also confirmed that as of 2021, UK visitors to the EU will have to pay 7 euros (about $8) for the European Travel Information and Authorization Scheme (ETIAS), which can be bought online ahead of travel. This will last three years and ensure smooth entry at EU borders and airports, similar to the current ESTA scheme many tourists use to travel to the United States.