THE case for a hard Brexit is “dead in the water,” Nicola Sturgeon has insisted after a leaked UK Government analysis concluded Britain would be worse off whatever deal is struck with Brussels.

The First Minister, describing the revelations, as a “watershed moment” in the Brexit process, declared: “Time is running out and the chaos in Downing Street must end.

“Theresa May must face down the hard Brexiteers around her and put jobs and living standards front and centre of the Brexit negotiations by remaining in the single market and customs union.”

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Ms Sturgeon insisted that the Whitehall now must be published in full.

Her comments come as MPs prepared for an Urgent Question in the Commons about the leaked analysis called for by Labour and the SNP.

As the Prime Minister prepared to fly out on a three-day trade mission to China, opposition MPs were demanding the public were entitled to know the true cost of leaving the EU.

Mrs May leaves behind her a Conservative Party in turmoil amid deepening unrest among MPs over the direction of the talks with Brussels.

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The mood in the party will not have been helped by the leak of the latest economic impact assessment, drawn up for the Department for Exiting the EU, showing growth would be lower under a range of potential scenarios.

Even if the UK were able to negotiate a comprehensive free trade agreement - as the PM hopes - it estimated growth would be down five per cent over the next 15 years, according to the document seen by the BuzzFeed News website.

That would rise to eight per cent if Britain left without a deal and was forced to fall back on World Trade Organisation rules.

Alternatively, if the UK were to retain access to the single market through membership of the European Economic Area the loss would be just two per cent.

“This devastating leak is a watershed moment in the Brexit negotiations,” insisted Ms Sturgeon.

“For months, Theresa May’s Government have refused to produce any detailed analysis of the potential impact of various Brexit scenarios; now we know why they have so desperately engaged in a cover-up.”

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She pointed out how the UK Government’s own analysis mades clear that leaving the EU would, in all circumstances, harm the economy of every nation and region in the UK, underlining the case that remaining in the single market and customs union was the best way to minimize that economic harm.

“When the Scottish Government published our own impartial analysis a few weeks ago, showing an extreme Brexit could cost each person in Scotland £2,300 a year, the Tories accused us of scaremongering; now we find out that behind the scenes they agreed with us.

“The Prime Minister must now agree to publish this analysis in full and any other analyses which they are concealing from the people of these islands.”

She added: “The case for a hard Brexit is dead in the water and it is no wonder that, with every passing day, the extremists in the Tory Party are becoming ever more desperate. Time is running out and the chaos in Downing Street must end.”

Responding to the leaked analysis Iain Duncan Smith, the former Conservative leader, told BBC Radio 4's Today programme: "The timing in this is highly suspicious in the sense that suddenly in the midst of all this conversation about the European Union we have a leaked document.

"But,I would observe that almost every single forecast coming from Government, and most of the international organisations, has been completely wrong. We should take this with a pinch of salt," he added.

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But Tory donor Charlie Mullins, who backed Remain in the EU referendum, called for the PM to quit.

“It's obvious now there is no future for her. She needs to go. And as I'm being honest, the quicker the better. We have all lost confidence in her and it is bad for business for her to linger on in there.

"We all know she is going to go. She may as well just book the removal lorry and move on," he added.

The PM raised the leaked analysis at the start of the Downing Street Cabinet, stressing to her colleagues that it "purported to show" the economic impact of various Brexit scenarios.

Mrs May pointed out how this was in fact "initial work, not approved by ministers," which only considered off-the-shelf options; options, which the Government has already rejected. The leaked analysis did not cover the bespoke deal ministers are looking for. No other minister spoke on the subject at Cabinet.

Later, her spokesman made clear the leaked report was a "draft, incomplete document, not signed off by ministers" but was part of a number of analyses the Government was engaged on.

He declined to say whether or not it would be published once it was completed. He also declined to say if the document had been a freelancing operation by a civil servant or if there would be a leak inquiry.

The spokesman stressed how the Government's main focus was getting a bespoke deal on Brexit, which would benefit not only the UK but the EU as well.