Video report by ITV News Political Correspondent Carl Dinnen
Ms Flint, who lost her seat to the Conservatives in the election, told Sky News that Ms Thornberry made disparaging remark about voters, accusing her of telling a colleague: "I’m glad my constituents aren’t as stupid as yours”.
But Ms Thornberry, who denies making such remark, said on Monday morning she has been forced to take legal action over the claims she says are "simply untrue".
"I've said to her please withdraw, I'll give you til the end of the day, and she hasn't so I've had to go to solicitors."
She added: "People can slag me off, as long as it's true I'll take it on the chin, but they can't make up s*** about me and if they do, I have to take it to the courts".
Former minister Ms Flint, who lost her seat in the former stronghold of Don Valley, had been highly critical of “ardent Remainers” within the party, citing among them Ms Thornberry and shadow Brexit secretary Sir Keir Starmer, for having “contributed to sacrificing 59 seats” during Thursday’s election.
She told Sky’s Sophy Ridge On Sunday: “I don’t believe anybody who have been the architects of our European policy in the last few years is credible to be leader – I don’t think they can win back these seats.
“Keir Starmer led us to a policy that did not listen to Labour leave voices who urged caution, he led us down the path of a second referendum, and I’m afraid Emily Thornberry did as well – she said to one of my colleagues, ‘I’m glad my constituents aren’t as stupid as yours’.”
Ms Thornberry roundly rejected the allegation as the battle to succeed Mr Corbyn began bringing longstanding tensions within the party to the surface.
“The quote Caroline attributed to me today is a total and utter lie,” the Islington South and Finsbury MP tweeted.
“I’ve never said that to anyone, nor anything like it, nor would I ever think it.
“Whatever our differences, let’s not sink into that gutter.”
The row comes amid a race to succeed Jeremy Corbyn as Labour leader, with Ms Thornberry believed to be considering a bid.
Others thought to be in the running are Sir Keir Starmer, Rebecca Long-Bailey, David Lammy, Jess Phillips and Angela Rayner.