Emily Thornberry, a potential Labour leadership candidate, has threatened to sue her former colleague Caroline Flint for “making up shit” about her.

Thornberry said she had consulted lawyers after Flint, who lost her Don Valley seat last week, claimed on live television that Thornberry had once said to a fellow MP in a leave-voting seat that she was “glad my constituents aren’t as stupid as yours”.

Thornberry told Sky News: “I’ve contacted her and I’ve said to her: please withdraw, I’ll give you until the end of the day. And she hasn’t. So I’ve had to go to solicitors.

“People can slag me off as long as it’s true, I can take it on the chin. But they can’t make up shit about me - and if they do, I have to take it to the courts.”

She added: “It’s ridiculous, absolutely ridiculous. I have better things to think about than people going on television and making up shit.

“I have no idea what’s in her head. All I can tell you is that you can’t go on national television and make up shit and not expect to be taken to the courts, and that’s what I’m afraid I’m having to do.”

Flint made the comments in an interview with Sky on Sunday, in which she argued that the next Labour leader should be someone who had not pushed the party towards a pro-remain position, as Thornberry and Keir Starmer had.

On Sunday Thornberry said of Flint’s claim: “This is a total and utter lie. I have never said this to anyone, nor anything like it, and I hope, needless to say, it is not something I would ever think.”