A little known rule could render passports invalid in the event of a no-deal Brexit.

In the past weeks thousand of people across the country received a text message advising them that their passports will be invalid if we crash out of the EU without a deal.

But many believed it was a scam.

The text directs people to a website checker which tells you if you need to renew.

"I received a text on January 29 telling me my passport would be invalid and I should renew," one reader the Oxford Mail. "But I thought it was a scam. But at the weekend I checked on the gov.uk website, and my passport would be invalid for travel to any of the EU's Schengen Zone countries [including holiday destinations such as France, Greece and Spain]."

But whilst the focus has been on those whose passports run out in the next six months, a little-known rule could means yours is invalid even if it has more than six months to spare.

Most passports last for a standard 10 years. But some British passports last for 11 years - because the past government added up to nine months to a new passport if you renewed your old one before you needed to.

But the Schengen Zone rule won't count that extra time - so if you renewed your passport on March 29,2009, your passport will become invalid on March 29, 2019, and the extra nine months won't count.

Figures obtained by Which? show that around 3.5 million current British passports will not comply with the validity rules.

The passport office takes about three weeks to process a renewal application, which means today is the cut-off date if passport-holders want to receive a new one by March 29.

BACKLOG FEARS

The Home Office says a pre-Brexit rush may cause delays to passport renewals, so people whose passports are about to run out are urged to get it sorted as soon as possible.

It costs £75.50 for a new adult passport and normally there's a three-week turnaround.

In an emergency there's a one-week turnaround option, which costs £142. But as a No Deal looks increasingly likely, turnaround times could double.

WHAT ABOUT PETS?

Pet passports and UK driving licences may also become invalid in Europe if there's a No Deal. You'll probably need an international driving licence.

Go to gov.uk for passports, pet passports and driving licences.