THERESA May has been accused of constitutional “trickery” as she today makes a last-gasp attempt to get her Brexit deal through the Commons on the day Britain was due to leave the EU.

The Prime Minister’s tactical “wheeze” to win a vote on just one half of the UK-EU deal – the Withdrawal Agreement or the divorce part of the agreement - faces a high wall of resistance as Labour branded it the “blindest of blind Brexits” and the SNP and DUP pledged to oppose it too.

During First Minister’s Questions, Nicola Sturgeon told MSPs the whole Brexit process was now, thanks to the Tories, a “complete and utter mess” and insisted stopping it altogether “must now be our top priority and moreover that is now possible".

She mocked the PM, noting: "Theresa May must be the only leader in living memory who has tried to fall on her own sword and has managed to miss."

But Donald Trump, addressing reporters outside the White House, came to her defence, describing Mrs May as “strong, tough and she’s in there fighting”.

The US President added: "She's a very nice lady. She's a friend of mine. I hope she does well. I hope the Brexit movement and everything happening there goes very well.”

Today’s latest Commons debate and vote on Britain’s departure from the EU will take place as outside Parliament pro-Brexit campaigners stage a mass protest at the delay to coincide with the original exit day of March 29.

The Government’s attempt to pass just the first part of the deal is a bid to buy time.

The EU offer to grant an extension to May 22 is specifically conditional on MPs passing the “Withdrawal Agreement” this week.

The Government hopes that if it is successful, then it can use the next eight weeks or so to win support for the second half; the Political Declaration on the future relationship with the EU.

If it fails, then the extension only runs for the next two weeks until April 12.

On Monday, MPs will again take control of the parliamentary agenda to try to find consensus around an alternative plan.

Under the 2018 EU Withdrawal Act, Brexit can only happen if MPs approve both the Withdrawal Agreement and the Political Declaration together.

But the Government suggested if MPs approved the first part of the deal, then it would bring forward a Withdrawal Agreement Bill, which could be rewritten to obviate the need for a third meaningful vote. Any detailed discussions on the future relationship could then take place after Brexit happened.

However, the chances of Mrs May getting the Withdrawal Agreement through appear slim given it contains the controversial Irish backstop customs arrangement, which continues to represent the biggest obstacle to support from Northern Ireland’s Democratic Unionists and Brexit-backing Tories in the anti-EU European Research Group.

In a 20-minute phonecall with the PM yesterday, Jeremy Corbyn confirmed Labour MPs would not back the Government move.

A party spokesman said: "He made clear Labour will not agree a blindfold Brexit to force through Theresa May's damaging deal, which would leave the next Tory Party leader free to rip up essential rights and protections and undermine jobs and living standards."

Mrs May’s dramatic offer to resign on Wednesday night to enable a successor to take over the second part of the Brexit process in negotiating the future UK-EU relationship had limited success. While some Tory Brexiteers swung behind her plan as the “least worst option,” others did not.

And the DUP reaffirmed its own opposition to it as it would carve out Northern Ireland from the rest of the UK by putting a regulatory border down the Irish Sea.

After John Bercow, the Commons Speaker, made clear the Government could not bring back a substantially unchanged Brexit plan - given it had been heavily rejected twice - ministers and officials had to scratch their heads to find a way to present it again.

Having a debate and vote on just the Withdrawal Agreement was ruled in order by Mr Bercow and Andrea Leadsom, the Commons Leader, urged all MPs to “support it and ensure that we leave the EU on May 22, giving people and businesses the certainty they need".

But the Government’s highly unusual move sparked an outcry on opposition benches.

Labour’s Valerie Vaz accused UK ministers of treating MPs in an “absolutely disgraceful” way, which underlined their disrespect for Parliament.

"This is the Government playing games,” declared the Shadow Leader. “Parliament, our constituents, future trading partners and the country will not countenance this."

Ben Bradshaw, the former Labour Cabinet Minister, warned the Government against using “more attempted trickery or potentially illegal trickery” by separating out the Withdrawal Agreement from the Political Declaration.

His Labour colleague, Mary Creagh told MPs: "This is an extraordinary and unprecedented reverse ferret of the commitments that have been given by ministers to this place...that we should have our say on both items together.

"And is it not extraordinary that we have this coming right on the day when we know there are going to be far-right demonstrators gathering in Parliament Square?" asked the Yorkshire MP.

The SNP’s Pete Wishart was scathing, insisting Westminster’s processes over Brexit were in an “absolute guddle”.

He explained: “It has almost got to the point in this House where it is so broken and the debate is so corrupted that we are now having debates through Points of Order. I can barely imagine that we have reached that stage just now; it demonstrates how badly broken things are.”

The Nationalists’ Shadow Leader added today’s sitting was “all about the Government’s latest wheeze to get their doomed Brexit deal through”.

He added: “They are inviting us to consider the Withdrawal Agreement without the Political Declaration attached. It is a meaningful vote but it is a sort of Schrödinger’s meaningful vote; it is both alive and dead at the same time.”

Labour’s Chris Bryant described today’s vote as a “meaningless” one and warned the Government: “The one precedent I'm absolutely sure the House always abided by in the past and will abide by probably tomorrow is when the Government comes up with a policy, a change of mood, change of style, a different way of doing business which is too clever by half, it always loses."

Ms Vaz also warned the Government motion could risk seeing the law "being broken" as it did not meet the requirements of the 2018 European Union Withdrawal Act.

But Geoffrey Cox, the Attorney General, sought to reassure MPs, saying: "When the House listens to the rationale behind it, when it hears the full context of it, I'm sure the House will accept it is not only perfectly lawful, perfectly sensible and is designed to give this House an opportunity of availing itself of a right the European Union has given to us to avail ourselves of an extension until May 22.

"The view of the Government is simply we could not let the time limit expire at 11pm tomorrow of allowing this House the opportunity of availing itself of that right. It is perfectly reasonable and it is perfectly lawful," he insisted.

Sir Edward Leigh, the former Conservative minister, said his colleague Sir Oliver Letwin, the architect behind the process of so-called “indicative votes,” would take control on Monday to pursue Brexit deal alternatives if the Withdrawal Agreement were not approved before then.

"There is nothing to stop him now under our procedures from whittling down the options to one option, which almost certainly given the results last night would be permanent membership of the customs union.

"There is nothing to stop him putting this in a Bill, there is nothing stopping him making that an Act of Parliament and then the choice will be between - and I say this to my colleagues - permanent membership of the customs union or a general election," added the Lincolnshire MP.

Mrs Leadsom replied: "I certainly feel this House needs to agree to fulfil on the 2016 referendum and the Prime Minister's deal offers the means by which to deliver on that referendum but at the same time for those, who don't want to leave the European Union, to offer the closest economic and security partnership, it is a compromise and I do urge colleagues right across the House to back it."

At Holyrood, the FM clashed with Jackson Carlaw, the interim Scottish Conservative leader, over the issue.

Ms Sturgeon declared: "This whole process thanks to the Tories is now such a mess that stopping Brexit altogether must now be our top priority.”

There was criticism of SNP MPs, who voted against the proposal for the UK to have a customs union with the EU despite the party having long argued in favour of a soft Brexit, which would mean the UK staying within both this and the European single market.

Mr Carlaw accused the Nationalist leader of attempting to stock up "faux outrage, grievance" on her "single-minded obsession" of independence.

"When it came to the crunch the First Minister whipped her MPs against supporting her own policy of a customs union," declared the Eastwood MSP.

"Isn't it the case that what Scotland saw yet again yesterday is then when push comes to shove for the SNP it is not about finding a solution to Brexit it is about pursuing their independence obsession."

But Ms Sturgeon hit back, saying: "For two long years when stopping Brexit didn't seem possible the SNP argued for single market/customs union membership compromise; that was ignored by the Tories and indeed everybody else.

"Now that option, which is the minimum you would need to protect Scotland's interests, wasn't actually on the ballot paper last night. That said, over the next few days we will continue to work across Parliament for a compromise of that nature, if that proves to be the only alternative to a hard Brexit."

In other developments -

*Speaking at the British Chambers of Commerce conference, David Lidington, May's de facto deputy, pleaded with MPs to back the Brexit deal, saying: “Politicians of all political parties have a duty to put the national interest first so that we can put this controversy behind us and move on to a bright future for the British people."

*Boris Johnson, who indicated he would back the PM’s deal following her resignation announcement, has reportedly told friends: “It’s dead anyway.”

*A poll suggested Brexit was now the most important issue for Scots when considering if the nation should be independent. Some 43 per cent cited this as the key one ahead of 42 per cent who regarded the economy as the most pressing factor.

*Mark Francois, vice-chairman of the European Research Group of Tory Brexiteers, told BBC TV: “I told my whip the other day I wouldn’t vote for it if they put a shotgun in my mouth.”

*Another Conservative Brexiteer Andrew Bridgen argued parliamentary arithmetic meant the PM’s deal stood no chance, saying: “Twenty Conservatives and the DUP are not going to vote for it. It is not going to go through.”

*The SNP’s Neil Gray said the Government’s failure to replace Sarah Newton, who quit a fortnight ago as Disabilities Minister over Brexit, was a "shameful indictment" of the Tory administration and was evidence that it was “consumed” by it.

*MPs will on Monday afternoon debate the online petition - signed by nearly 6 million people - calling for Brexit to be scrapped.