In 2026, flying with an expired passport in the UK is a "hard-fail" for international travel, but a grounded possibility for certain domestic or Common Travel Area (CTA) routes. As of February 25, 2026, the UK has strictly enforced the Electronic Travel Authorisation (ETA) scheme, requiring valid digital documentation for almost all travelers. For flights between the UK and the Republic of Ireland, carriers like Aer Lingus now mandate a valid passport or Irish Passport Card for all passengers, effectively ending the use of expired documents for those routes. However, for domestic flights within the UK (e.g., London to Edinburgh or Belfast), airlines like British Airways or easyJet may still accept an expired British passport as a form of photo ID, typically if it is within five years of its expiry date. A supportive peer warning: because acceptance is at the "Carrier's Discretion," you should always check with your specific airline first. For the most frictionless experience, renewing your passport is the only "Gold Standard" way to ensure you aren't denied boarding.