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Hays Travel, the UK's largest independent travel agent, has said people with older red passports should check both the issue date and expiry date on their document.
This is because some passports may appear to have 'extra months' on, but if they were issued before the UK left the EU, these additional months are 'no longer valid'.
Hays Travel Plympton, the Plymouth branch of the travel agent, shared a statement on Facebook to warn travellers of the issue.
It said: "Important passport update changes: due to changes in validity since leaving Europe, your passport is now only valid for 10 years from the issue date on your passport.
"If your passport was issued prior to the UK leaving the EU with additional months on, those extra months are no longer valid."
Giving an example to illustrate the point, the post continues: "Passport issue date 21st Jun 2012, passport expiry date 21st Dec 2022.
"Your passport expiry date is now the 21st Jun 2022."
It adds: "Most countries in the EU require that your passport is valid for six months on the date of return from your holiday, some countries it's three months, meaning the last date you could return on the above example would be the 21st Dec 2021."
It also directed people to the government website, where there is a passport checker.
The government advises that travellers should always check the entry requirements of the country they're visiting to find out whether or not they need to renew their passport, as rules can vary from place to place.
"If you're using your passport as ID to visit Ireland, you do not need to renew it as long as it's valid for the whole of your stay," it advises on its website, where people can also check their passport for travel to Europe.
To use the passport checker, you'll just need to know when your passport was issued and when your passport expires, along with basic information about yourself and your planned trip.
Of course, on top of all this, there's also the various coronavirus restrictions that you'll need to bear in mind, as some countries remain on the UK's red list - which means travel to these areas is strongly advised against.
Similarly, other countries may have closed entry to arrivals from certain places.
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