Need some very quick advice as trying to book flights: my daughter (UK/US) is looking to fly from UK to Mexico direct flying and entering on her UK passport. The return trip is via JFK so she would need to fly out on and enter the US on her US passport which will only have 3 months left on it and then fly out again on her US passport to the UK because as a US national she must enter and leave the US on her US passport. Our question is as she is a dual national, can she fly out of the US on her US passport and tell the airline (British Airways) that she is a dual national with a valid UK passport that she will use to enter the UK on so that they don't ask her why she doesn't have the new ETA visa and do not stop her from boarding the flight from JKF to Heathrow? Also, is she able to change her passport details in her online booking from UK to US for her return trip easily?

I've been mulling this over for myself too. I'm wondering if you can just use your UK passport with the airline reservation and hand over your US passport at immigration check points?
Last year I booked my flights from usa to LHR using my US passport , exited US with US passport and upon arrival in LHR I asked if I could just enter using my UK passport. UK border control said I could but it might raise flags because I had left one country and entered another using 2 different passports. So it made me wonder if I was conflating immigration/airline ticketing when they really were separate systems?

Also hoping someone can come along and help out.

She won't be able to check in for the return flight in Mexico until BA are satisfied that she is entitled to enter both the US and the UK legally.
She may need to present both passports at the airport to achieve that.

She does not TELL BA that she is a British national, she carries her British passport with her.

This is just an airline issue. It is perfectly fine, but she may not be able to check in online. The check-in staff will ask for proof she can enter her destination(s) and she can show both passports. They will enter what they need accodingly.

Qantas staff once had me wait 5 mins for them to dsappear and "link" my passports, but that didn't happen again after a renewal, so it seems to be a one-off.

The US does not have exit checks, so nothing needs to be done to leave the US "on" any passport.

that’s not exactly accurate, they don’t have a physical CBP officer looking at your passport, but when you checkin for an international flight you input a passport, if not saved to your profile airlines may verify it at bag drop or if carry on only the gate before departure. That information is on the passage manifest which is shared with US and UK authorities. That’s how the US knows when a VW visitor has left (and not overstayed) and the UK knows who is arriving…

Right but there are no consequences for a US citizen not having a US passport on their outbound passenger manifest.

Exactly.

When traveling to the UK from the USA I always present my UK passport to the airline as that is what they are focused on. They face heavy fines if they board a passenger who doesn't have valid documents for arrival in their destination country. I carry my US passport as well.

But she would be checking in in Mexico and will need to present her US passport to prove she can first fly into JFK or should she just show both passports at that time before she flies out of Mexico?

Yes ....

Both.

Yes, carry both and present them as requested.

https://uk.usembassy.gov/worldwide-t...anuary-8-2025/

So, we need an answer to this - at what point do we prove UK citizenship ? I'm thinking when we book the flights because otherwise the airline is going to be requiring the ETA

When you check in. This has been the situation for Australia for a long time -- I book the ticket with my AU passport and they ask how I will be allowed in the US when I am flying back. I stopped over in NZ on the way back recently and it was also fine.

The airlines and immigration can handle this all just fine. The only time it can get sticky is with online check-in, so you may just need to go to the counter.