Emily Thornberry says quote attributed to her by Caroline Flint is a "total and utter lie"

The war of words broke out after Caroline Flint – who lost the once safe Labour seat of Don Valley last week – claimed that Ms Thornberry had told a colleague: “I’m glad my constituents aren’t as stupid as yours”.

However Ms Thornberry, a close ally of Jeremy Corbyn and a possible contender for the Labour leadership, dismissed the remarks as senior party figures turned on each other over the scale of the party’s humiliating and historic election defeat.

“The quote Caroline attributed to me today is a total and utter lie,” tweeted Ms Thornberry. “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.”

Ms Thornberry represents the Islington South and Finsbury – the constituency adjacent to Mr Corbyn’s own North London seat.

She first came to prominence during the Rochester by-election in 2014 when she was forced to resign from the Labour front bench for posting a derogatory tweet about a house in Kent that was adorned with three England flags and had a white van parked on its drive.

Her tweet came after Ms Flint, a former minister who had represented her South Yorkshire seat for over 21 years, stepped up her criticism of Labour’s had “ardent Remainers” for their role in the party’s worst electoral performance since the Second World War.

She cited Ms Thornberry and Sir Keir Starmer, the Shadow Breixt Secretary, for having “contributed to sacrificing 59 seats” because of their desire for a second referendum on EU membership.

Ms Flint, one of the few Labour MPs to have backed Theresa May and Boris Johnson’s Brexit deals, 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 Flint’s comments prompted Labour’s Richard Burgon to concede on the same programme that the party’s position on holding another EU referendum “did fail”.

However the Shadow Justice Secretary and Leeds East MP, who remains a key ally of Mr Corbyn, blamed Labour’s defeat on the poll being the “Brexit election” and defended the party’s socialist manifesto.

On Labour’s Brexit position, he maintained: “I think it was right to attempt to bring the country together on that basis. Did it fail? We’ve got to be open, it did fail. It was a disastrous election result and for that we are truly sorry.”

He also criticised the Sun and Daily Mail newspapers for conducting a “character assassination” against Mr Corbyn. “I think the biggest mistake the Labour Party made was perhaps underestimating the desire for people who had voted Leave to leave the European Union,” Mr Burgon added.

He backed Rebecca Long-Bailey, the Shadow Business Secretary, to succeed Mr Corbyn and said he is “considering” running as her deputy.

Meanwhile John McDonnell, the outgoing Shadow Chancellor, apologised for his role in Labour’s campaign. He also said the next leader should be a woman - which would make her the first to lead the party - and said it was “most probably time for a non-metropolitan” candidate as he said “we need a northern voice”.