An urgent warning is being made to British travellers heading to the Europe to check their passports before departing, or else they may be turned away.

This year, millions of British citizens are expected to travel across several destinations across the European Union, including popular vacation destinations like Spain, France, and Italy. Before you leave for the airport, though, be sure you understand one very important guideline. British citizens now have blue passports instead of red ones, but only the red passports will be subject to the little-known EU 10-year expiry date requirement as a result of Brexit. Red passports are only accepted by EU countries if expiration is under ten years.

Although a passport’s standard expiration date is ten years, older renewals frequently have the remaining expiry time from the original document—up to nine months—added to the expiry date of the new document. This indicates that certain passports are ineligible for travel to the EU since they may be as old as 10 years and 9 months at the time of writing. British passports that are valid for travel must not only be no more than ten years old, but also remain valid for three months after the date of your intended return to the UK.