I can find no text anywhere that says it’s prohibited. I would imagine, even though other countries prohibit it, it would cause with issues with persons for example who are British citizens by operation of law (i.e., by decent), but have never obtained a US passport, or may not even be aware they have British citizenship.

Unofficial source FWIW:

https://uk-eta.com/uk-eta-faq/

Home office publication (basically iterates it’s “not required” for British citizens, but no mention of prohibited):

https://assets.publishing.service.go...dance__11_.pdf

In the FWIW department, the US passport requirement is found at section 215(b) of the Immigration & Nationality Act [8 USC 1185(b)].

In doing research, it should be noted that the custom among immigration cognoscenti is to use Act citation rather than Code citation. In a long ago gift to immigration lawyers, the implementing regulations of the 1952 Act generally track Act citation. Most immigration lawyers will probably give you a blank stare if you talk about section 1255 rather than section 245. In further confusing numerology, many have a habit of using form numbers so that 245 may be referred to as 485.

Australia is the same.
I've been following this expecting to see the UK introduce the same rule, but as yet I've seen nothing to say that dual citizens can't get an ETA on their other passport. Its bound to come though, its a huge loophole in their scheme.

I'll also be surprised if they get the scheme live by January, but I guess miracles happen!

I wonder how transportation companies will cope?

For example, earlier this year I did a back to back cruise, New York to Southampton back to New York. I only provided my US passport for this trip since there were no requirements to enter on my UK passport. I know the cruise lines only have one passport field, so I can only provide one passport number, yet they would need my UK passport to provide to UK, and my US passport for returning to NY. Will they refuse me boarding due to not having an ETA or ESTA depending on which passport they record?

I can't see anywhere that UK citizens need a UK passport to travel to the UK. From what I read, if you are a UK citizen who does not want to stump up 10 quid for an ETA then you will need to show a UK passport.

We used to do a lot of cruising and gave up trying to provide 2 passports on departure for trips to the UK and used to only provide our US passport but that was a PITA when we moved back in 2016, and got cheap tickets on a Princess “repositioning cruise”. We did actually get them to accept both passports at check-in but after we arrived in Southampton UK border control came on board to check all non-UK passports. We were given a time-slot to present our passports while British Passport holders were allowed to depart. When we asked if we could leave we were told that our US passport details had already been transmitted so UK officers would expect to see us so we wouldn’t be allowed off. When we did see the UK agent he laughed and waved us through.

Hmm. Not sure about this.

Once I had US citizenship, I never attempted to either leave or enter the US without a valid US passport (and used it to buy my plane tickets, too) so I can't be 100% sure. However, there are most certainly exit checks when flying at least (been a long time since I crossed a US land border) in the sense that when you check in for your flight you will be put on the flight manifest and these are provided to the customs and border people. This is why, when you come back in, an immigration officer knows when you left and where you went. It also means that Immigration will know if a non-citizen overstays (again, not sure how land borders are now managed).

I also hugely doubt you will ever get anywhere near a US air border without a valid passport (unless you have a visa/esta of another sort). The airlines require one, they generally will not let you even attempt to board, get a boarding pass, or even confirm your flight unless you can show that you have the right to enter the US. So the second part of your post above is moot really.

Having said all of that, I'm still at a loss as to why somebody entitled to a passport wouldn't just get one. Not having one seems like a totally unnecessary complication. They aren't that expensive.

The link provided above says that the US is one of many exempt countries anyway.

Since I became a US citizen, I have always traveled back and forth over the Atlantic flying on my US passport, but using my UK passport in person when entering the UK (and the US one when leaving to return to the US). There was never any problem is showing a UK passport on entry to the UK although my travel record held the US passport. I don't think they even check the manifest, they just check the passport, and in you go. See no reason for that to change really.

In my experience airlines were never interested in seeing my US passport when leaving, I did try from time to time but they were focused on the destination country. I used to travel from the US to UK a lot for work, and then after retiring we spent up to 6 months at a time in the UK in any one year so I always presented my UK passport to the airline at US departure and on entry to the UK. Coming back was the same process in reverse.

During the 1990s Newcastle airport had exit checks for a while, where it was similar to Australia (been in and out there 4 times). I would show my US passport at the airline check-in desk and then my UK passport at UK border control to prove that my stay had been legal.

I've never tried leaving the US on a UK passport once I had a US one. I always booked round trips so I used my US passport, which I thought a) was required and b)would be needed to be my passport of record for the return flight so that everything matched. Matching became a very big deal as part of security. Nobody ever mentioned a specific passport by name when leaving - it never came up because my tickets were connected to that passport so that's what I gave them. Before that I used my UK passport, and my greencard for re-entry.

We have several cruises per year, usually departing from Florida, NYC or LA using our US PP. if disembarking in the UK, we use our UK PPs for entry. Other ports depends on whether they require visas. For example, we left NYC and disembarked in Rio. USC’s require a visa to enter Brazil, UKC’s do not, therefore we used our UK PPs to enter Brazil.

Edit: Same applies when entering a country by air.

same here, it was just that one cruise I mentioned that where we were delayed because UK border control came aboard after we docked. Usually we just disembarked and chose the UK passport line.

No problem if your elapse time was within the normal visitor visa timelines, and definitely easier.

In 2011 we left the USA in March, with a return in November so a US passport would not have worked. We didn’t spend all the time in the UK but spent a couple of months in Ireland as well. We did similar in 2013, and in 2014 we used our UK passport with 6 month visa to Australia. In each case the airline was more concerned with our destination validation than knowing if we were “leaving” on a US passport.

How do you mean your US passport wouldn't have worked? Where wouldn't it have worked? To leave the US? Seems odd?

I believe at the times mentioned above the maximum time allowed in the UK as a tourist from the USA was 6 months. Certainly in 2016 when I was moving back to become tax resident I needed to enter the UK on my UK passport. Same goes for 2017 and 2019 when I left the USA with no return ticket.