Theresa May has already “thrown in the towel” on key parts of Britain’s negotiations with the EU over Brexit, a Labour MP has said.

Chris Leslie, the former Shadow Chief Secretary to the Treasury, said the Prime Minister’s decision to publish her plan for Brexit would “expose” her failings in deciding to take Britain out of the EU’s single market.

Ms May announced during Prime Minister’s Questions that the Government will publish its plan for Brexit as a White Paper to be debated by MPs and Lords.

It comes after the Supreme Court ruled that Parliament must be allowed to vote on whether the UK should invoke Article 50 – the mechanism by which countries begin their withdrawal from the union.

Labour sources told The Independent they were expecting the paper to be published as early as Thursday, with debates to begin next week. The Prime Minister has promised to trigger Article 50 by the end of March and therefore wants to get parliamentary proceedings under way as quickly as possible.

Ms May was only publishing the paper because she had been forced to by Conservative backbenchers, Mr Leslie claimed.

Supreme Court Brexit Challenge Show all 13 1 /13 Supreme Court Brexit Challenge Supreme Court Brexit Challenge People wait to enter the public gallery outside the Supreme Court ahead of the challenge against a court ruling that Theresa May's government requires parliamentary approval to start the process of leaving the European Union, in Parliament Square, central London Reuters Supreme Court Brexit Challenge Gina Miller, co-founder of investment fund SCM Private arrives at the Supreme court in London on the first day of a four-day hearing Getty Supreme Court Brexit Challenge A man waves the EU flag in front of the Supreme Court Getty Supreme Court Brexit Challenge Satirical artist Kaya Mar poses with two of his paintings in front of the Supreme Court Getty Supreme Court Brexit Challenge Pro-Europe protestors dressed as Supreme Court Justices stand outside the Supreme Court ahead of the first day of a hearing into whether Parliament's consent is required before the Brexit process can begin. The eleven Supreme Court Justices will hear the government's appeal, following the High Court's recent decision that only Parliament can trigger Article 50 Getty Supreme Court Brexit Challenge The eleven Supreme Court Justices will hear the government's appeal, following the High Court's recent decision that only Parliament can trigger Article 50 Getty Supreme Court Brexit Challenge Businesswoman Gina Miller arrives at the Supreme Court ahead of the first day of a hearing into whether Parliament's consent is required before the Brexit process can begin Getty Supreme Court Brexit Challenge Attorney General Jeremy Wright arrives at the Supreme Court in London EPA Supreme Court Brexit Challenge Protesters outside the Supreme Court in London, where the Government is appealing against a ruling that the Prime Minister must seek MPs' approval to trigger the process of taking Britain out of the European Union PA wire Supreme Court Brexit Challenge A protesters wearing a judge's wigs and robes stands outside the Supreme Court ahead of the challenge against a court ruling that Theresa May's government requires parliamentary approval to start the process of leaving the European Union, in Parliament Square, central London Reuters Supreme Court Brexit Challenge A protester holds up a placard outside the Supreme Court ahead of the challenge against a court ruling that Theresa May's government requires parliamentary approval to start the process of leaving the European Union, in Parliament Square, central London Reuters Supreme Court Brexit Challenge Pro-Europe protestors dressed as Supreme Court Justices stand outside the Supreme Court Getty Supreme Court Brexit Challenge A man waiting to enter the public gallery waves a European Union flag outside the Supreme Court ahead of the challenge against a court ruling that Theresa May's government requires parliamentary approval to start the process of leaving the European Union, in Parliament Square, central London Reuters

He told Sky News: “Theresa May is making out this is no big deal but it is something she’s been forced to concede, not least by her own benches – if you look at [Conservative MP] Anna Soubry just yesterday pushing on this particular issue.

“It now means there are further concessions the Prime Minister could be pushed into [making] because she’s worried about the numbers in Parliament.

Mr Leslie criticised the Prime Minister’s decision to withdraw Britain from the single market – something he said the British public had not voted for.

“I think the White Paper will expose what a bad plan she has and how it’s a hard Brexit plan”, he said.

“We’re not even going to try to salvage the single market membership, which I think she should have tried to do. She’s accepted [the EU’s] red lines”

“[Remaining in the single market] is vital for our economy. The white paper will expose that her plan has thrown in the towel on that option already.”

Theresa May confirms there will be a white paper on Brexit

Conservative MP Jacob Rees-Mogg said the failure to remain in the single market was David Cameron’s responsibility.

He said: “This is what David Cameron should have been in during his re-negotiation, and didn’t deliver. We have now voted to leave the European Union, not to remain in a marginally reformed European Union.

“The Remain campaign including David Cameron and George Osborne consistently said that voting to leave the European Union meant leaving the single market.