Traveling with two passports US to UK document question (visa?)

Either way you need an ETA, see the official government website, it requires one for both US and Italian citizens: https://www.gov.uk/guidance/check-when-you-can-get-an-electronic-travel-authorisation-eta

An ETA is not a visa, which is a separate and different process. You should get the ETA for the passport you intend to use when traveling.

"If I enter the UK with my U.S. passport, I need to get a visa for my visit."

For regular visitors from the USA, visa or ETA: https://www.gov.uk/check-uk-visa/y

Maybe I’m wrong but shouldn’t you use one passport for your travels instead of mingling two? Sounds like a problem in the making.

I have dual US/EU citizenship and was told by U.S. Immigration authorities to always use my U.S. passport both in departing and reentering the US or there will be complications.

You should use your U.S. Passport to enter the UK.
No Visa is required of US passport holders entering the UK. An ETA is required to enter the UK.

The UK is no longer part of the EU. This means your Italian (EU) Passport would ALSO REQUIRE AN ETA to enter the UK.

For a vacation in th UK, your Italian passport offers no benefits. Both the US and Italian passports require that you obtain an ETA for a regular tourist visit of 6 months or less in the UK. Or a visa for any other type of visit. And as you already know, you need to use your US passport to leave and return to the US.

Really, for practical purposes, the only time your Italian passport offers any benefits is for entering and exiting the EU.

I’m sure you already know this, but just in case…
If and when you enter or exit any Schengen Agreement EU country, you will need to use your Italian EU passport. It’s the law.
At US airport check in I use my US passport. Arriving in Europe or Iceland, I use my European passport. And reverse it going to the US.

I also travel on two passports sometimes and agree with what others have said above. For the avoidance of doubt- you can travel on two different passports on the same roundtrip ticket (one going out and a different one going back). You can give the info for either passport when you are booking the tickets. But when you check in you will give them the passport you are actually using to travel that day, and if needed they will update it at the check-in desk for you. They do this all the time and it's not an issue.

That’s interesting Elizabeth. We did not know this. My husband has 2 (US and Croatia) while I just have the US. He always carries both with him but only presents the US when we travel to Europe. Even within Europe.
We’ll have to rethink this strategy on our next trip to Italy in April.

Similarly if one of the nationalities held is British then these days you are supposed to enter the UK on it.

Thanks, all.

You can book the ticket with either passport. The passport number on the ticket doesn’t have to match the one you use at every border.

In practice, use your U.S. passport when leaving and re-entering the U.S., and show your Italian passport when entering the UK to avoid the visa. Airlines are used to this with dual citizens. Just make sure you carry both passports and show the right one at each step.

If you have a US passport, you don't need a visa for the UK for tourism purposes.

If it's for work or school, both US and Italian passports need a visa in the UK.

You can book the ticket with either passport. The passport number on the ticket doesn’t have to match the one you use at every border.

In practice, use your U.S. passport when leaving and re-entering the U.S

This is correct, with the exception that leaving the US, there is no exit immigration. To a degree, the airline does not care what valid passport you show for documentation, though sometimes you may want to inform them that you have a second passport. Two examples would be, if traveling to the UK, and you have a UK passport, so no ETA, you need to show your UK passport in order to clear documentation. This will also be the case whenever the Schengen area implements ETIAS and you hold a Schengen member passport, or an EU member passport. The other instance is if you will be spending more time in a country than what the Visa Waiver program allows. So go to a Schengen country for more than 90 days, you will need to show your Schengen/EU member passport.