Caroline Flint’s claim was rejected as “a total and utter lie” by shadow foreign secretary Ms Thornberry.
But Ms Flint told The Independent that she stood by her story, which threatens to undermine any bid to succeed Jeremy Corbyn by Ms Thornberry, who has not yet confirmed whether she plans to stand in the coming leadership contest.
The former Don Valley MP said that senior Labour figures like Thornberry and shadow Brexit secretary Sir Keir Starmer who argued for a second referendum were not “credible” as future leaders.
And she also said that the leader should also not be “Corbyn without a beard”, warning that the agenda he followed had put off a lot of traditional working-class voters.
Ms Flint was one of the loudest voices in the parliamentary Labour Party arguing for the result of the 2016 referendum to be respected.
She told Sky News’ Sophy Ridge on Sunday: “I don’t believe anybody who has been the architect of our European policy in the last few years is credible to be leader – I don’t think they can win back these seats.”
And she added: “I don’t believe there’s any credibility for these people. 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’.
“I’m sorry, it’s not acceptable. We are going to have to really look at what might make a candidate for leader who can at least steer us to a better place over the next few years.”
Responding to Ms Flint’s comments, Ms Thornberry said: “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.”
Ms Flint, who served as MP for Don Valley for 22 years and was a minister under Tony Blair and Gordon Brown, said that since 2010 Labour had paid too much attention to voices from London and the university cities and not enough to its heartland areas in the Midlands and the north.
“I’m trying to stem my anger, but I’m emotional because I do believe that in the pursuit of Remain a number of people who have been ardent Remainers in our party, on our front bench – people like Kier Starmer, people like Emily Thornberry, but many others like Hilary Benn, Yvette Cooper – they have contributed to sacrificing 59 seats … but don’t worry, we’ve got Putney and Canterbury,” she said.
“With all due respect to the Labour MPs for those constituencies, I don’t believe that was worth it.”
Asked if Jeremy Corbyn was taking enough personal responsibility for the party’s defeat, she replied simply: “No.”
And on the question of who should take over as leader, she said: “I don’t think it should be anybody who has had a hand in our Brexit strategy over the last few years, and I don’t think it can be Corbyn without a beard. I think that would be the wrong move as well.”
Islington South and Finsbury MP Ms Thornberry previously lost her post in Ed Miliband’s shadow cabinet in 2014 after sending a tweet about a house draped in England flags with a white van parked outside, which was widely criticised for snobbery.