ETA necessary?

No, they removed any requirement for passengers in transit to have an ETA. You will stay airside and not go through Immigration.

But you will go through security. All connecting passengers whose flights originated outside the UK, must go through airport security again.

Just follow the signs for "Flight Connections" and it will lead you to this. Also make sure you have your boarding pass for the second flight.

No need for an ETA if just connecting flights and not going through immigration.

Paul and Frank, thanks so much! That’s what I found going through LHR in March, but changing terminals to Belfast. Made sure I had my boarding pass!

That is pretty bad that British Air doesn't know how to answer American passengers on this rather basic question !

I got the same answer from British Airways. I had asked that same question to BA, KLM and WestJet because even though I was on one ticket, I'd need to change terminals, and none of the airlines would confirm that I didn't need an ETA.

In April I was checking in for my connecting flight from Paris to London with onward travel to Chicago. The BA agent insisted I needed an ETA.

Cathy,
Your statement above appears incorrect.
If you are transiting at LHR to a flight to Belfast, you will need an ETA, as Belfast is in Northern Ireland, that is part of the UK.

Did you have an ETA at that time?

Traveler99…
Thank you…maybe I wasn’t clear…my flight to Belfast was in March, and I had my ETA because I was entering the UK. My question was for a flight this September, where I am the only one of four traveling with an ETA…we will transit through LHR to our connecting flight to Venice. My question was whether our friends need to get an ETA to simply transit within Terminal 5. Sorry if two separate trips wasn’t clear.

as long as all of you have one ticket for the journey from home to Venice via Heathrow you can remain airside and they don't need an ETA for that leg. Yours will still be valid but unnecessary in this case.

Thanks, Nigel…I was more concerned about our friends. This seems to be a fairly straightforward issue, and I explained that we would stay airside, but the BA agent said, “Well, the rules can change”…. Sigh.

Well technically, that BA agent is correct - things can change (indeed, they have before, could again). But the best we can all do is prepare to follow the rules as they exist now (assuming we can find a clear consensus on them...) and be monitoring the situation in case something does change again.

The OP's friends should be aware of all this; if they're not experienced travelers and/or not good at sussing out details and following requirements, this would be a good time to start getting better about such things. I'm sure plenty of people blithely just show up with a passport and assume all is covered - an honest mistake since that's how things have been before, but not much of an excuse.

Which raises the question: No doubt UK authorities receive many unprepared "what's an ETA?" arrivals (or those who shrug and say, "well, the airline said I wouldn't need that") every day. How dire are the consequences for those visitors? Surely they have some provision to do a (probably expensive) on-the-spot ETA (otherwise, the border force jails would be overflowing...). Note that I'm not looking to test that theory (got my ETA a couple days ago), but still curious about what happens and how big of a hassle comes with it. I assume you don't just get an immediate flight back to your home country to do the paperwork and start over...

I’ve had my ETA since February, my husband applied for his last night and the approval email came through one minute later! Our friends will have theirs as well. Thanks, all!