> if you enter a country whose citizenship you have you must always enter with the travel document or proof of citizenship of that country. no exceptions.
Completely untrue. I have done so perfectly legally.
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It's not a law in either the US or UK as far as I know, but both countries always got pissed off at you if they figured it out. I know because I've been lectured by border-officers on both ends before :)
"Pissed off" here meaning that you were likely to get "randomly selected" for secondary screening.
It absolutely has been the convention that you use the local form of identity if you have one. This ETA issue is just them pushing that a bit harder.
> It's not a law in either the US or UK
This changed on Feb 25, 2026 for the UK. :)
UK citizens must now enter on their UK passport (or a citizenship certificate thing + foreign passport), and are not eligible for visa waiver programs (because they're only eligible for people using certain passports, which UK citizens obviously now can't be using).
I was announced in Nov '25, and has cause a mad scramble for lots of people as the passport office has been massively backlogged by the predictable queue of people needing passports suddenly, when they didn't need them before.
It's 100% illegal to present a non-US passport at the US border if you are US citizen.
The law is 8 U.S.C. 1185 - "it shall be unlawful for any citizen of the United States to depart from or enter, or attempt to depart from or enter, the United States unless he bears a valid United States passport."
In the past, the penalty for violating this has generally just been "a stern talking to," like you said. But no guarantees on that.
Can confirm that UK immigration was annoyed by it, but it was perfectly legal nonetheless.
This changed on 25/2/26. See my comment above.