Travelers with UK ETA multiple passports should apply using the passport they will actually use to travel to the United Kingdom.
A UK ETA is linked to the passport used in the application, so the passport used for the application should match the passport presented during travel.
If one of your passports is a British or Irish passport, you do not need to apply for a UK ETA.
Instead, you should use that passport to prove your right to travel to the UK.
If you do not have a British or Irish passport, you may apply using a passport that is eligible for a UK ETA.
Before traveling, make sure your flight details, passport details, and UK ETA application information are consistent.
If you renew your passport or decide to travel with a different passport, check whether your existing UK ETA can still be used before your trip.