In case of a No deal Brexit:
Firstly, travellers will need to have at least 6 months left on their passport.
Secondly, any extra months on a passport over 10 years may not count towards the 6 months needed. A passport may have extra months if the holder renewed their previous passport before it expired.
The Home Office will send around one million text messages to passport holders who may need to renew their passports early. Not everyone provides a mobile number, and contact details may have changed, so even those who do not receive a message should check their passport.