The Shadow Foreign Secretary hit out at rumoured plans by frustrated Labour MPs to create a new pro-Remain party called Back Together.
She said those fighting to for a second referendum or to block Brexit altogether were a result of “the normalisation of dishonesty”.
And she issued a direct attack on Chuka Umunna, who was linked with the rumoured Back Together party, accusing him of having “got it all wrong”.
Ms Thornberry said in Westminster yesterday evening: “I’m talking about our political culture as a whole: a culture where, for example, on one side of the house of Commons we have the European Research Group, and on the other we have what we’re being told is a Back Together group.
“Both of them engaging in earnest debate in the house and in the media about how to deliver the best deal for Britain but the truth is that neither group actually cares what kind of deal we get and neither is genuinely interested in an honest debate about the merits.”
She said what Brexit rebels in both parties really wanted was to “reverse the democratic decision of the referendum” and urged them to “be honest and simply say so”.
The MP for Islington South and Finsbury then hit out at Mr Umunna specifically, accusing the MP for Brixton of having “got it all wrong”.
She said: “When I heard Chuka Umunna last year talk about the main lesson to take from the Blair government is don’t over-promise and under-deliver, I think Chuka had got it all wrong.
“I think the lesson from the Blair government isn’t to water down the promises you make.
“You ought to be keeping the promises you make, and stick to the standards that you set.”
Mr Umunna, who has denied he was behind the plan to create a new pro-Remain party, took umbrage at Ms Thornberry’s comments.
He told the Independent he thought her comments were “insulting and patronising” and accused her of betraying her party in order to secure a “good headline”.