Emily Thornberry has said she is prepared to take legal action against Caroline Flint for telling “extraordinary lies” and “making up shit” about her.

Flint, a former Labour MP who lost her Commons seat at last week’s general election, said Thornberry had told another colleague: “I’m glad my constituents aren’t as stupid as yours.”

Thornberry, the shadow foreign secretary, told Sky News she had given Flint until the “end of the day” to withdraw the accusation.

“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 said.

Flint made the claim in an interview with Sky News’ Ridge on Sunday programme over the weekend.

Thornberry, who is believed to be eyeing a bid to succeed Jeremy Corbyn as party leader, told ITV News she missed the programme at the time because she was cutting back a hedge “as a bit of therapy”.

Flint has been contacted by HuffPost UK for comment.

Flint’s old seat of Don Valley overwhelmingly voted Leave in the 2016 referendum. Thornberry’s Islington South and Finsbury backed Remain.

Labour’s leadership contest is due to formally get under way in the New Year. But the list of candidates is already emerging.

Rebecca Long-Bailey, the shadow business secretary, is seen to have the support of many in the current leadership.

Angela Rayner, the shadow education secretary, is expected to run, as is Keir Starmer, the shadow Brexit secretary.

Wigan MP Lisa Nandy is a likely candidate, along with backbencher and ardent Corbyn critic Jess Phillips.