Labour and former Tory MPs join in bid to force through extension with amendment on ‘super Saturday’

Boris Johnson’s plan to push through a Brexit deal on Saturday looks likely to be frustrated after an alliance of Labour and former Tory MPs united behind a plan to force a new extension.

After clinching a last-minute deal in Brussels on Thursday by agreeing to a customs border in the Irish Sea, the prime minister had hoped to frame the rare Saturday sitting of parliament as a dramatic “new deal or no deal” moment.

But despite positive reactions from Bank of England governor Mark Carney and declarations of support from several Labour MPs, Johnson’s plans were rocked on Friday by a cross-party group led by Oliver Letwin and Hilary Benn.

The Conservative and Labour MPs will table an amendment allowing parliament to withhold its approval until the legislation to implement Brexit has passed.

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With Labour expected to back it, as well as rebel Conservatives including Philip Hammond and David Gauke, it appears likely to pass.

Letwin has insisted that his plan is not to block a deal. But the move would mean that even if Johnson secured the support of a majority of MPs, he would fail to pass the test set by the Benn act for Brexit to go ahead by 31 October, which sets a deadline of 11pm on Saturday.

The prime minister would then be legally obliged to request an extension from the EU27 – something he has repeatedly insisted he will not do.

Johnson would not need to make use of the extension, if he could pilot his Brexit bill through all its parliamentary stages before 31 October. But it would decisively remove the risk of a no-deal Halloween Brexit.

News of the amendment came as a serious blow to Johnson, with strategists meeting on Friday night to try to decide how to respond if it passed.

Despite the fact that several of its supporters repeatedly backed Theresa May’s deal – unlike Johnson himself – No 10 sources sought to claim it was a thinly-veiled bid to block Brexit.

“We can all see what this is about,” one said. “It’s being exploited by a bunch of people who are finding ever more inventive ways of not delivering on Brexit. They are not prepared to vote for a deal.”

Downing Street has repeatedly insisted it will comply with the law – but also that Johnson is determined to leave on 31 October, come what may. If he fails to send the letter, or seeks to frustrate the law in any other way, he is likely to face a legal challenge.

Meanwhile, as the scramble to win the knife-edge vote unfolded on Friday, several Labour MPs and previously unreconciled Tory Brexiters declared their backing for the agreement. But the numbers for and against Johnson’s deal appeared too close to call.

Johnson told the BBC on Friday night: “There’s no better outcome than the one I’m advocating tomorrow.”

Urging MPs to support him, he said: “I just kind of invite everybody to imagine what it could be like tomorrow evening, if we have settled this, and we have respected the will of the people, because we will then have a chance to to move on.”

Melanie Onn, the Labour MP for Grimsby, was one of several who announced they were prepared to support a deal, in the face of intense pressure from senior party figures to reject it.

In an article for the Guardian, written with Conservative MP Victoria Prentis, Onn said that the pair intended “to implore our colleagues to use this unique chance to help us move on, and get back to helping our constituents”.

“The risk of letting this final shot at a deal slip through our fingers is too great,” the pair add. “Our collective hope rests on brave Labour MPs, and indeed others, who can see that.”

Tensions among political leaders ratcheted up ahead of Saturday’s vote as the Scottish National party leader, Nicola Sturgeon, pointed the finger at the Labour leader for not being tough enough in opposing Johnson’s deal.

Sturgeon accused Jeremy Corbyn of presiding over an atmosphere within Labour where his MPs believe that if they defy the three-line whip and back the PM, they will not face any consequences such as being ejected from the parliamentary party.

She said: “My growing suspicion is that Labour will allow Boris Johnson to get his deal over the line.

“It seems to me that it is possible we have a situation where Labour have an official position of opposing this deal but are giving nods and winks to so-called Labour rebels in the hope that there’s enough of them that allow it over the line.”

Meanwhile, Carney used a US television appearance to say the deal agreed between London and Brussels would boost growth and could be a turning point for the global economy.

In an interview with Bloomberg TV in Washington, he said: “It is good news that there is an agreement. I would expect the economy to pick up from quite a subdued pace.”

Carney said the EU referendum had been the start of a period of trade uncertainty that had caused the global economy to slow. “The UK may have led the world into this and we may be leading the world out of this,” he said.

Labour has vehemently rejected Johnson’s deal. Keir Starmer warned that it would “pave the way for a decade of deregulation” and “give Johnson licence to slash workers’ rights, environmental standards and consumer protections”.

The scope for the Letwin amendment to succeed will partly be framed by the response of European leaders. Jean-Claude Juncker suggested in Brussels on Thursday that a request for an extension could be rejected – but other senior EU figures did not follow his lead.

If the amendment passes, instead of a yes or no to Johnson’s Brexit deal, MPs would then be given the opportunity of voting on the amended motion – which would say they have not yet approved the deal.

Some of those backing the idea, including Letwin, would like to support Johnson’s deal, but had become alarmed that it could pass on Saturday – allowing Johnson to escape the strictures of the backbench Benn act – but then fall at a later stage in the ratification process, leading to a no-deal Brexit.

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The move came as Johnson and his team launched a charm offensive, aimed at tempting potential waverers from across the political spectrum to support his agreement, which paves the way for a significantly looser future relationship with the EU.

In a last-ditch attempt to win over vital Labour votes to secure his Brexit deal, he released fresh details of government commitments on workers’ rights and environmental standards.

He is also offering a parliamentary lock on the future partnership with EU which will allow MPs a vote on the government’s negotiating objectives and a meaningful vote once talks have concluded.

As part of his raft of new commitments, Johnson has promised to maintain high standards of workers’ rights in the withdrawal agreement legislation when it goes before MPs.

A minister would also have to explain to MPs where any new bill could affect employment rights and compatibility with the EU. There would be an obligation on the government to report regularly on new EU measures and whether it would mirror them. MPs would also get to vote on this report.

In terms of environmental policy, Johnson has said he will look at cooperating with European agencies where it supports the national interest.

He also pledged not to open up British markets for goods that do not meet the UK’s high standards, and that the UK will not sign trade deals that export the environmental cost of its consumption to the rest of the world.

A Downing Street spokesperson said: “The UK has a long and proud tradition of leading the way in workers’ rights and environmental protections where we have always set a high standard. We recognise that MPs want to see these hard-won rights protected, not weakened by our departure from the EU.”

If the vote on Johnson’s deal goes ahead without any amendments, the decision could be agonisingly tight, with the decisions of the handful of MPs yet to indicate a preference potentially swinging the result.

With the threshold needed for a Commons majority of 320, once Sinn Féin MPs and the Speaker and his deputies are excluded, a Guardian tally of the two sides put them virtually neck and neck.

A key constituency yet to declare was pro-deal Labour MPs informally led by Caroline Flint, who have been heavily courted by No 10.

These calculations would be completely different if the Letwin amendment was to pass, with a series of potential opponents sufficiently reassured to back the government plan.

However, on the other side of the equation, some hardline Tory Brexiters might become more wary of supporting a plan which would inevitably require another Brexit extension.