In his first major interview since talks with the Government began almost five weeks ago, Sir Keir said more than two-thirds of the party’s 229 MPs could scupper the plan. He told the Guardian: “A significant number of Labour MPs, probably 120 if not 150, would not back a deal if it hasn’t got a confirmatory vote. “If the point of the exercise is to get a sustainable majority, over several weeks or months of delivering on the implementation, you can’t leave a confirmatory vote out of the package.
“I’ve made it clear that at this stage, at this 11th hour, any deal that comes through from this Government ought to be subject to the lock of a confirmatory vote.”
The issue of a confirmatory referendum has been an internal battleground within Labour ranks, with Sir Keir pushing for one but shadow business secretary Rebecca Long-Bailey, also part of the negotiating team, less keen.
But Sir Keir highlighted how the party lost 200 lost seats in this month’s council elections, which he said were a sign Labour was losing the trust of Remain as well as Leave voters.
With less than two weeks before the European elections, the shadow Brexit secretary urged Labour Remainers tempted to vote for the Lib Dems or Change UK that only Jeremy Corbyn’s party could deliver a fresh referendum.
Sir Keir added: “There is concern in leave areas about whether some of our voters might vote for other parties.
“But I think there is an increasing concern that some of the Labour Remain voters might not vote Labour.
“It is very important that we learn those lessons.”
Sir Keir, who was a human rights lawyer for 20 years, said he believed the European election was more than a proxy Brexit vote, but about the UK’s values “of dignity, equality and non-discrimination”.
He said: ”I did not think in my lifetime we’d actually be having an argument about those values.
“It’s really important we make the case that this is not the country of Nigel Farage and Tommy Robinson.
“That intolerance and hatred and division is not representative of our country. If they win, that is what they are representing.”
But talks are running out of time if any Brexit deal is to be agreed without UK MEPs having to take their seats in Brussels, and Theresa May is also under pressure from Tory MPs to reveal this week when she will quit.
Sir Keir said Brexit “cannot be divorced” from the Tory leadership battle.
He suggested Mrs May could not set a date without making things “even more tight”.
Sir Keir also said he would not be afraid to pull the plug on talks as early as this week if Mrs May did not budge on her red lines.
He added: “I do think we do probably in the coming days need to make that assessment”.