Boris Johnson’s hopes of finally obtaining parliament’s consent on Saturday for a historic deal to take the UK out of the European Union look set to be thwarted by an extraordinary bid by MPs to delay the crucial “meaningful vote”.

Ahead of the highly unusual weekend sitting of the Commons, the prime minister was pulling out all the stops to secure the 320 votes he needs to claim victory in what was expected to be a razor-edge ballot, with the balance held by wavering Labour MPs, hardline ERG “Spartans” and expelled Tory rebels.

But an amendment to his motion tabled by exiled Conservative Oliver Letwin threatens to withhold MPs’ approval until the full legislation to implement the deal is put into law.

With Labour expected to back the amendment, it seems almost certain to pass, after being selected for debate by Commons speaker John Bercow.

The move – designed to avoid no-deal Brexit if the legislation is amended by Brexiteers or fails to complete its passage through parliament by Halloween – would force Mr Johnson to request an extension from Brussels beyond 31 October and could delay the meaningful vote for weeks.

Downing Street indicated it could try to pull back from Mr Johnson’s motion if Letwin's amendment passes, which would deny him the chance to claim he has parliament’s support for the last-minute deal struck with EU leaders on Thursday.

And if the motion fails, he could then face a non-binding vote on a second referendum put forward by Labour backbenchers Peter Kyle and Phil Wilson with the aim of demonstrating that a majority in the Commons back a public ballot on any future change to the relationship between the UK and EU.

The drama came as the result of the vote – if it is held – remained impossible to call, with No 10 buoyed by winning over some Labour rebels while whittling down opposition in its own ranks.

Up to nine Labour MPs are now expected to vote with the Conservatives, after Ronnie Campbell, Sarah Champion and Melanie Onn – who all represent Leave-backing seats – spoke out.

With at least 12 Tory opponents of Theresa May’s deal announcing they would swing behind the prime minister’s reworked version, his chances of victory were growing.

Johnson made a last-minute effort to lure Labour MPs with a set of concessions on workplace and environmental safeguards announced late on Friday night.

But the package - including a promise that ministers will report to parliament on whether the UK will match future EU employment regulations and a pledge not to open UK markets to countries with low environmental and animal welfare standards - was dismissed by Labour’s Laura Pidcock as an “empty gesture not worth the paper it’s written on”.

Labour was locked in a civil war over the punishment to be meted out to its rebels, with Momentum founder Jon Lansman demanding they be expelled, while Jeremy Corbyn insisted they would not.

Supreme Court verdict: Boris Johnson prorogation unlawful Show all 16 1 /16 Supreme Court verdict: Boris Johnson prorogation unlawful Supreme Court verdict: Boris Johnson prorogation unlawful The Supreme Court has ruled against Boris Johnson by declaring his government unlawfully shut down parliament for five weeks EPA Supreme Court verdict: Boris Johnson prorogation unlawful Lady Hale handed down the historic verdict, hailed by opposition MPs and anti-Brexit campaigners. She said all 11 judges were unanimous in deciding that the case is “justiciable”, so the government loses that part of the argument. “The court is bound to conclude therefore that the decision to advise Her Majesty to prorogue was unlawful because it had the effect of frustrating or preventing the ability of Parliament to carry out its constitutional functions.” EPA/Supreme Court Supreme Court verdict: Boris Johnson prorogation unlawful Gina Miller and her team react outside the Supreme Court are the verdict. She said: “Today is not a win for any individual or cause. It is a win for Parliamentary sovereignty, the separation of powers and independence of our British courts. Crucially, today’s ruling confirms that we are a nation governed by the rule of law, laws that everyone, even the Prime Minister, are subject to." PA Supreme Court verdict: Boris Johnson prorogation unlawful A person dressed as a caricature of British Prime Minister Boris Johnson in a prison uniform stands outside the Supreme Court AP Supreme Court verdict: Boris Johnson prorogation unlawful Labour leader Jeremy Corbyn called for Mr Johnson to “consider his position” following the landmark decision, while Lib Dem leader Jo Swinson said he was “not fit to be prime minister” Getty Supreme Court verdict: Boris Johnson prorogation unlawful Scottish National Party (SNP) Westminster leader Ian Blackford raises his arms as he comes outside. He said: "This is an absolutely stunning judgement by the Supreme Court today." He went on to say, "we all want to get back to work, and quite frankly, on the back of this, Boris Johnson must resign immediately." AFP/Getty Supreme Court verdict: Boris Johnson prorogation unlawful Crowds outside celebrated the verdict AP Supreme Court verdict: Boris Johnson prorogation unlawful Delegates at the Labour party conference applaud after hearing the news AFP/Getty Supreme Court verdict: Boris Johnson prorogation unlawful Speaker John Bercow said MPs must now “convene without delay” and confirmed the Commons would return at 11.30am on Wednesday AFP/Getty Supreme Court verdict: Boris Johnson prorogation unlawful Gina Miller said: "“As a result of this judgment, Parliament is open, it was never prorogued. I urge MPs to get back to work immediately.” AP Supreme Court verdict: Boris Johnson prorogation unlawful MPs Anna Soubry, Liz Saville Roberts and Caroline Lucas, together with SNP leader Ian Blackford, react. Green MP Caroline Lucas has said the Supreme Court’s decision is “just the start” Reuters Supreme Court verdict: Boris Johnson prorogation unlawful “The UK deserves a Prime Minister and a Government who act with honesty, integrity and in a manner consistent with our constitution, at all times." AFP/Getty Supreme Court verdict: Boris Johnson prorogation unlawful Anti-Brexit supporters react as they gather outside the Supreme Court in London, Tuesday, Sept. 24, 2019 as it makes it's decision on the legality of Prime Minister Boris Johnson's five-week suspension of Parliament. (AP Photo/Frank Augstein) Frank Augstein AP Supreme Court verdict: Boris Johnson prorogation unlawful The SNP’s Joanna Cherry QC says: “Boris Johnson’s position is untenable and he should have the guts to resign.” PA Supreme Court verdict: Boris Johnson prorogation unlawful epa07865617 Protesters react outside the Supreme Court after the result of a hearing on the prorogation of parliament, in London, Britain, 19 September 2019. The Supreme Court ruled that the suspension of parliament by British Prime Minister Boris Johnson was not lawful. EPA/NEIL HALL NEIL HALL EPA Supreme Court verdict: Boris Johnson prorogation unlawful Tom Tugendhat MP returned to the chamber in the House of Commons within minutes of the Supreme Court ruling PA

The Letwin amendment has been tabled to close a loophole in the Benn Act, designed to ensure Mr Johnson must seek to delay Brexit if a deal has not been approved by 11pm on Saturday.

The loophole would allow the prime minister to escape the commitment of seeking a delay if the vote on his deal passes, even though a full withdrawal agreement bill is also needed to ratify the deal – leaving the UK open the risk of a crash-out on 31 October if the bill failed to pass in time.

A furious government source described the amendment as something that “looks as reasonable as possible, but is designed to frustrate the process”.

Joe Owen, Brexit programme director at the Institute for Government, told The Independent: “If the Letwin amendment passes, MPs will only have considered the matter, they will not have approved the deal.”

Crucially, because the Benn Act would not have been satisfied, Mr Johnson would have to request an Article 50 extension – breaking his pledge to lie “dead in a ditch” rather than do so.

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It is also possible that Mr Johnson would be barred from another attempt to stage a “meaningful vote”, forcing him to move directly to the weighty withdrawal agreement bill.

There are certain to be a blizzard of amendments, including for a Final Say referendum, and the bill will take several weeks to pass, going beyond the 31 October scheduled departure date.

But Sir Oliver played down the implications of his amendment, insisting MPs would still have the opportunity to “show in principle they are in favour” of the deal – which he himself supports.

He described it as an “insurance policy which means, if something goes wrong with the legislation, then we ensure the country will still be in the EU after 31 October”.