Shadow Brexit secretary Sir Keir Starmer has devised six tests the final agreement has to meet – widely regarded as a pretext to vote down the deal when it comes to the Commons later this year.
The tests include the impossible-sounding proviso that the final EU agreement must provide the “exact same benefits” as Britain’s current relationship with the bloc.
But, at a Chatham House event today, Ms Thornberry suggested the final deal would probably be so vague that it would pass the tests anyway.
She said: “What’s the nature of the divorce? And I think if past evidence of the last few months is anything to go on, it’s going to be a ‘blah, blah, blah’ divorce.
“It’s not going to actually make any decisions, it’s going to continue to kick things down the road.
“We don’t seem to have come to any difficult decisions at the moment.
“The difficulty is going to be with the meaningful vote in October, which we have secured, is that, what is it we are going to be agreeing on?
“We have our six tests. If you hold up ‘blah, blah, blah’ to the six tests, it will probably pass it.”
Labour sources have been quick to play down her words, stressing no decision would be taken on the vote until the terms of the deal were laid out.
But Sir Kier and his allies are said to be fuming at the comments.
And ultra-Remainer Mr Umunna, who heads the Open Britain campaign, took a swipe at his colleague.
He said: “It is extraordinary and unacceptable that the shadow foreign secretary seems to be suggesting that some ‘blah blah’ from the Government will be enough to secure Labour’s support to write the Government a blank cheque for Brexit.
“It’s an old-fashioned idea but it is the job of the opposition to hold the government to account and that is what our members expect to see rather than blasé chat about ‘blah blah’ on the most important issue facing the country.
“The public will rightly take a very dim view of the Labour frontbench joining arm in arm with the likes of Jacob Rees-Mogg and other Brextremists to vote for a Brexit which will cost jobs, damage living standards and leave our public services with less investment.”
This week, Sir Kier said Labour would also try to rewrite the Government’s Brexit legislation to prevent the UK leaving the EU without a deal if Parliament rejects any agreement Theresa May strikes with Brussels.
The opposition hopes to build a coalition of peers and MPs to reject the “take it or leave it” approach on offer from ministers which would see a vote against the final agreement interpreted as a decision to back a “no deal” Brexit.
Sir Keir said: “If Parliament rejects the Prime Minister’s deal, that cannot give licence to her, or the extreme Brexiteers in her party, to allow the UK to crash out without an agreement.
“That would be the worst of all possible worlds.
“That is why in the coming days, and working with others in the Lords and the Commons, Labour will ensure that an amendment is tabled to the EU (Withdrawal) Bill to strengthen the terms of Parliament’s meaningful vote.
“Should the Prime Minister’s deal be defeated, it must be for Parliament to say what happens next, not the executive.
“Our amendment would make it clear that, should the Prime Minister’s deal be defeated, it must be for Parliament to say what happens next, not the executive.”
Under the plan, if Parliament rejects the deal, MPs would vote on a Commons motion instructing the Government what to do next.
Labour’s position on Brexit has come under scrutiny following Owen Smith’s dismissal from the shadow cabinet after calling for a second referendum.
The party has not definitively ruled out calling for a second public vote although deputy leader Tom Watson said it was “highly unlikely” that Labour would call for another referendum.