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 starting on January 8, 2026.

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.

An ETA is required to enter the UK starting on January 8, 2026.

Pretty sure that is a typo.

UK ETA went into effect Jan 8 2025