The shadow Brexit secretary Sir Keir Starmer made the announcement after speculation the party could abstain when the prime minister brings the Withdrawal Agreement Bill (WAB) to the Commons in the first week of June.
Last night, Jeremy Corbyn was facing a hostile response from backers of a Final Say referendum, including one senior minister, after his spokesman repeatedly refused to rule out the prospect of the party abstaining.
But speaking on Thursday during Brexit questions, Sir Keir warned Ms May against ploughing ahead with a critical vote without a positive resolution in the talks between Labour and the Conservatives – now in their seventh week.
“I’d have thought it was patently clear that if the prime minister’s deal is put for a fourth time, if it’s allowed, it will fail just as it has failed three times already,” he told MPs.
“But I want to make it clear that Labour opposes the idea of passing the Withdrawal Agreement Bill without an agreed deal – that would put the cart before the horse, and Labour will vote against at second reading on that basis.”
His remarks echoed those of Emily Thornberry, the shadow foreign secretary, who told ITV’s Peston the party will oppose the WAB, saying: “I’m the elected politician. I’m on the record: we’re not going to vote for the Withdrawal Bill if we don’t agree it, which we don’t unless they make changes which they haven’t.”
Sir Keir continued in the Commons: “How on Earth does the secretary of state think that a Bill to implement a deal that isn’t before the House can pass in two weeks’ time, or is about keeping the prime minister in office for another week to give her a lifeline for today’s meeting of the 1922 Committee?”
Brexit secretary Stephen Barclay, in his reply, said: “It will be an issue for the House to decide when it sees that legislation whether it does command a majority of the House.
“It may be that his personal position is that it’s irrelevant what’s in that text because he personally wants a second referendum.
“That’s not the basis on which the discussions have been held, that may be his personal position, it is not – as I understand it – the official position of the Leader of the Opposition.
“But it will be for the House to make a decision, and what the prime minister has made clear is there’ll be an opportunity to do that in the week after recess.”