THE UK Government is facing a dramatic showdown with Parliament this week as MPs face a potential “run-off” between opposition parties’ preferred soft Brexit alternative and Theresa May’s three times-rejected withdrawal plan.

Downing St was keeping its counsel until another round of so-called “indicative votes” takes place tonight before it sets out its next steps in the constitutional crisis gripping the country.

Tomorrow could be a key day in how things progress as it was confirmed the Prime Minister and her senior ministers will meet for five hours with a political Cabinet in the absence of civil servants from 9am until noon followed by a full Cabinet from 1pm until 3pm.

And it has emerged Mrs May is to hold a mass meeting with some of the 170 Conservative MPs who signed a letter to her demanding Britain leaves the EU on April 12 "with or without a deal".

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In what could turn out to be a significant development Labour announced it would be whipping its MPs to support the Common Market 2.0 option, promoted by former Conservative minister Nick Boles.

The SNP announced it too would be supporting the motion that offered a Norway-style soft Brexit.

The move indicates Labour is shifting its position towards staying in the single market and, crucially, accepting the free movement of people.

Mr Boles welcomed the move, tweeting: "It comes closer to Labour’s stated policy than any other Brexit compromise and now has a real chance of winning the majority that has eluded the PM."

Until now Labour’s policy has been for the closest possible alignment to the single market but not full membership like Norway, partly because single market membership would involve accepting free movement and so not having full control over immigration from the EU.

Emily Thornberry, the Shadow Foreign Secretary, said: “We find ourselves at the 11th hour or perhaps the 59th minute of the 11th hour, trying to pull something together because of the manifest failure of the government. And people need to compromise.

Asked if Labour was now willing to accept freedom of movement, she replied: “What we are trying to do is trying to pull the House of Commons together and what we are trying to do that will get maximum support because we don’t accept Theresa May’s deal and we don’t want no-deal.”

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A Labour spokesman later added: "In line with our policy, we're supporting motions to keep options on the table to prevent a damaging Tory deal or no-deal, build consensus across the House to break the deadlock and deliver an outcome that can work for the whole country."

SNP Westminster leader Ian Blackford said his party would also be backing the Boles motion, despite preferring the UK to remain in the EU.

"The key thing for us is that we stay in the European Union," Mr Blackford told the BBC. "We want to revoke Article 50. We want to put it back to the people in a People's Vote.

"But we will be prepared to compromise on the basis of protecting jobs, staying in the single market and customs union, so we will vote for the Boles amendment."

If all 35 SNP MPs fell in behind the plan tonight, it could win a majority unless seven to right or more Labour MPs rebelled against their party's whip.

Last week, the closest option to gaining a majority among several others, all of which were rejected, was the one put forward by Ken Clarke, the former Conservative Chancellor, for a customs union. It was rejected by just eight votes.

Mrs May has repeatedly dismissed the notion of a customs union as this would prevent Britain from having its own independent trade policy, something which she believes people voted for in the 2016 EU referendum.

The PM is hoping that, whatever materialises from tonight’s vote, it will focus the minds of those Conservative colleagues who have thus far failed to support her deal as – with no-deal off the table because Parliament would reject it – the choice, with just 11 days to the next scheduled exit day of April 12, will be between her deal and a long extension, which could produce a second referendum and the prospect of Britain staying in the EU.

But ardent Brexiteer Steve Baker said this morning he could vote against the Tory Government in a Commons no-confidence vote.

The former Brexit Minister told the BBC’s Politics Live that, if Mrs May backed the UK staying in a customs union and Labour brought forward such a motion, he might vote against the Government. “At this point, I can foresee no circumstances while, as a Conservative MP, I voted against the Government in a confidence motion. But we are approaching the point where the stakes are now so very high and so transcend party politics and what this country is about and the fundamental British value that political power rests on consent, that these things are coming on to the table.”

Mrs May spent the weekend trying to build support among MPs who could be won over with aides saying she was "100 per cent focused" on getting the result she needed.

She is also hoping to win over some Labour MPs with the promise, made during Friday’s debate, to accept a proposal made by six of them that Westminster should to have a greater influence in the future negotiations process with the EU.

But the Democratic Unionists remain staunchly opposed to the PM’s plan, believing it would undermine the integrity of the Union. Sammy Wilson, its Brexit spokesman, insisted the UK-EU deal would "take us away from the country that we fought to stay part of".

At the weekend, David Gauke, the Justice Secretary, warned the PM not to ignore the will of Parliament if it did swing behind a "softer" Brexit.

In an interview with the BBC, Julian Smith, the Government Chief Whip, took the unusual step of breaking his silence to argue that the parliamentary arithmetic meant a "softer type of Brexit" was inevitable.

He also berated his Cabinet colleagues, saying their divisions had produced the “worst example of ill-discipline in cabinet in British political history”.

This morning, Liz Truss, the Chief Secretary to the Treasury, pointed out that the will of Parliament so far had not been in favour of a customs union but added: “The answer lies in modifications to the Prime Minister's deal to be able to get that to have support.” She did not elaborate what the modifications were.

No 10 sources confirmed that, once again, Cabinet ministers would be told to abstain on the indicative votes while Conservative MPs would have a free vote.

The options on offer not only include a customs agreement with the EU but also a second referendum on any deal, a Norway-style “common market 2.0” deal and a power for MPs to vote to block a no-deal Brexit as well as scrapping withdrawal altogether.

Before any debate can take place, MPs will vote on a business motion so that the indicative votes process can proceed. Downing Street sources confirmed the Government would again whip against the motion as it believes Parliament taking over the Commons agenda risks creating a harmful constitutional precedent.

Meanwhile, in an open letter to MPs, the Chief Executive of Siemens UK urged them to unite around a "customs union compromise," which he said was "essential" to frictionless trade and would save companies billions each year.

Juergen Maier warned that the UK used to be a "beacon for stability" but was now becoming a "laughing stock".

His comments came after Michael Roth, the German Deputy Foreign Minister, lashed out at Brexit.

"Brexit is a big sh**show, I say that now very undiplomatically," declared Mr Roth.

He told a Social Democratic Party event in Berlin that "90 per cent" of the UK Cabinet had "no idea how workers think, live, work and behave".

Politicians, he argued, "born with silver spoons in their mouths, who went to private schools and elite universities" would not be the ones to suffer the consequences of Brexit.

Responding to his comments, Mrs May's spokesman said: "He answered his own question with the 'undiplomatic' comment. It is important that we get on with delivering on the verdict of the referendum for the British people. That's what the Prime Minister is focused on."

Speaking in Germany, Jean-Claude Juncker, the European Commission President, said it was time for MPs to make clear what they wanted.

"A sphinx is an open book in direct comparison with the British Parliament. We must get the sphinx to talk now. Enough of the long silence," he added.

*Andrea Leadsom, the Commons Leader, has written to MPs to tell them that the Commons will be sitting in the week commencing April 8 with a decision on whether it will sit in the week beginning April 15 to be “taken as soon as possible”. Westminster’s Easter recess had due to begin on Friday.