Flint is a widely respected MP from the north of England who was considered one of the party's best hopes to take the leadership should Corbyn quit,but lost her own seat after Labour's vote in the area plunged 17.8 per cent.
The explosive claim threatens to derail Thornberry's bid to replace Corbyn in next year's leadership election and is particularly damaging given Labour will need to rebuild trust in pro-Leave areas in middle and northern of England that swung heavily towards the Conservatives on Thursday.
A fuming Thornberry immediately denied the claim over the weekend but stepped up her response on Monday by threatening to sue.
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"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,"Thornberry said.
"People can slag me off. If some of it is true,I will 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 and we've all got better things to do.
"I hope she reconsiders and realises that this simply isn't fair or right and you can't just make things up on national television.