Nicola Sturgeon will today warn Theresa May her Brexit deal is doomed and she must look to a Plan B as the UK Government faces a constitutional showdown with Parliament.

The First Minister will also speak to opposition parties at Westminster about working together to reject the Prime Minister’s deal in the meaningful vote on December 11.

Ms Sturgeon, en route to UN climate change talks in Poland, will argue for a suspension of the Article 50 withdrawal process in order to pause Brexit and find “a better way forward”.

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However, Mrs May will try to shore up support for her deal by telling MPs it will allow Britain to strike "ambitious” trade deals around the world.

Updating the Commons on bilateral talks at the G20 in Argentina, she will say: “For the first time in more than 40 years we will have an independent trade policy.

“We will forge new and ambitious economic partnerships, and open up new markets for our goods and services in the fastest growing economies around the world.”

Ms Sturgeon is due to meet Mrs May as she faces a cross-party threat to hold ministers in contempt of Parliament for withholding the legal advice on her plan.

In a binding vote last month, MPs ordered the government to publish its full advice on the 585-page EU withdrawal deal, but it has refused, offering only a summary instead.

Attorney General Geoffrey Cox QC, who authored the final advice to the cabinet, will make a statement to the Commons this afternoon.

Shadow Brexit Secretary Sir Keir Starmer said if the advice was not released in full, Labour and other parties would start proceedings for contempt of parliament – something which could lead to hearings, delay the Brexit process and further damage Mrs May’s authority.

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"If ministers stubbornly refuse to obey the order of MPs then they risk triggering a historic constitutional row that puts Parliament in direct conflict with the executive,” Sir Keir said.

He also said that if Mrs May’s deal was rejected next week it was “inevitable” Labour would table a no confidence motion in the government to try to bring about a general election.

This is seen as unlikely to pass, and the debate would probably move on to a People’s Vote.

The government argues there is a long-standing convention that its legal advice is secret.

However, there is no legal ban on release, and the opposition and many Brexiter Tory MPs believe it is being withheld because it is so politically damaging to Mrs May.

The Sunday Times reported a leak of the advice showed it admitted the UK could be stuck in the Irish backstop arrangement “indefinitely” if future trade talks with the EU stall or fail.

And, last night, two Tory MPs who are also lawyers called for the advice to be released.

Simon Clarke and former minister David Jones said it was essential that parliament was fully informed about the implications of Mrs May’s deal before they voted on whether to accept it.

Ms Sturgeon said last week was a “watershed” in the Brexit process, with the government’s own analysis admitting any form of EU exit would make the country poorer.

She said: “With so much at stake for people’s jobs and living standards, it is vital that MPs come together to reject the PM’s deal, to rule out a No Deal Brexit and to secure an extension to Article 50."

SNP Westminster leader Ian Blackford said he expected an amendment to suspend the Article 50 withdrawal process to be tabled in the Commons within days.

Brexiter Environment Secretary Michael Gove said it would be “challenging” to get Mrs May’s deal through the Commons, and admitted it was not perfect, though still better than No Deal.

He said MPs might back a second EU referendum, but warned it could “rip apart the social fabric of this country”, with Remain voters feeling they were being treated as “thick”.