British Expats - What passport do you fly on?

The airline may also want proof of you being admitted on the other end prior to flying.

When I first got my Portuguese passport, I hadn't yet worked out which passport to book when putting in passenger information and so on... so when I booked my first trip back home after moving to the UK, I put my Portuguese passport information. I had a stopover in the US on the way back to Toronto. When I checked in at Heathrow, they asked to see my passport, so I handed over my Portuguese one, then they asked where my visa was - oops, didn't even think of that as I had "Canadian" in my head, but was living in the UK as an EU citizen. I said "Oh, I'm continuing to Canada, I'm a Canadian" and showed them my Canadian passport and they said "Oh, no problem then, make sure you use your Canadian passport to enter the US." And that was that.

My husband has American and British citizenship. He was back in the UK last month, so didn't bother bringing his American passport with him. When he left, he forgot that while he was flying directly to London, he was connecting in the US on the way back. Luckily he realised it as he was flying out so when he got to London he got himself an ESTA for the way back. (I think he said he didn't end up needing it because he had his PR card, but I can't remember offhand.)

Safest option is to just travel with the lot of it.

(That said, when I'm going just to the US, I don't bother packing my Portuguese passport, and Husband doesn't bother packing his British one, but does make sure he has his PR card!)