Proof that Momentum are giving Labour members details to Workers’ Liberty
Momentum — now Jeremy’s campaign — are running a phonebank app for people to log in and use.
Jill Mountford, who sits on Momentum’s committee and is a supporter of proscribed Trotskyist group Alliance for Workers’ Liberty, says she is organising these phone banks in Lewisham:

And here’s her Momentum branch saying the same thing:

Now, when asked if she was using Labour data, Jill said this:

She says she isn’t using Labour data, only Momentum’s data. That seems like it would not be a real issue.
But it doesn’t seem to be the case.
Here’s an email sent this week by Lewisham Momentum — Jill’s branch:

Again, Jill Mountford is the organiser. But there’s a handy link to the app.
Click the link and it gives some pretty strong rules, like how you have to be an eligible voter in the leadership contest:

But the crucial line is here, where it says they are calling Labour members not just Momentum members:

And here’s the email you get when you sign up.

It’s clear — it’s Labour members users are calling, not Momentum members.
That suggests it is Labour data, not Momentum data, that the app runs on. And why wouldn’t it? Campaigns are given access to Labour membership details — which is why you get called and emailed a lot.
And in any case, Jill Mountford is not an eligible voter.
So are Momentum giving your data to proscribed revolutionary groups?
They have a lot of questions to answer.