Economic growth will be 5 per cent lower after 15 years even if the UK manages to agree a comprehensive free trade deal with the, the study reveals.
Staying in the single market through membership of the European Economic Area would see growth slow by 2 per cent, while crashing out of the EU without a Brexit deal and relying on World Trade Organisation rules would knock 8 per cent off growth, according to civil servants.
Theresa May and ministers dismissed the study, leaked to the news website BuzzFeed, as an attempt to “undermine Brexit”, insisting it was incomplete “initial work” and did not examine the “bespoke” relationship the UK Government is seeking.
But it was seized upon by critics, who accused ministers of engaging in a “cover up” of bad news on Brexit.
“This devastating leak is a watershed moment in the Brexit negotiations,” Ms Sturgeon said.
“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.”
The figures revealed in the leak are broadly in line with the Scottish Government’s own economic assessment of available Brexit options, published earlier this year.
It prompted calls from the SNP for an apology by senior Tories including Ruth Davidson and David Mundell for dismissing the analysis published earlier this year. The Scottish Conservative constitution spokesman, Adam Tomkins MSP, branded the figures “scaremongering”.
The First Minister said: “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.”
Mrs May now faces a likely Commons bid to force the release of the analysis to MPs. Ms Sturgeon said 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 said: “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.”
Ms Sturgeon also took to Twitter to comment on the leaked report.
She said: “When @scotgov published analysis two weeks ago showing the cost of Brexit, some Tories accused us of scaremongering. Thanks to this leak, we now know the UK government is sitting on analysis which comes to precisely the same conclusions.”
Brexit minister Steve Baker said the report was a “preliminary attempt to improve on the flawed analysis around the EU referendum”, adding that it did not examine the UK government’s preferred option of a bespoke free trade deal.
Cabinet ministers were expected to have been briefed on the analysis this week, following Tuesday’s cabinet meeting.
Brexit Secretary David Davis was only made aware of its existence after news broke on Monday night.
His junior minister Steve Baker, who was summoned to the Commons on Tuesday afternoon to answer questions from MPs, said he was only made aware of the analysis that morning.
Publication prompted a furious row within government over where the analysis had come from, and who commissioned it.
An investigation has been launched into the leak, which Mrs May was forced to address at the start of cabinet.
A No 10 spokesman said: “The Prime Minister noted media coverage of a report purporting to show the impact of Britain leaving EU.
“The PM said this was initial work, not approved by ministers which only considers off-the-shelf scenarios.
“No analysis was made of the bespoke agreement we seek as a matter of Government policy.”
A Government source said the document was an “early draft” which needed further work before it was ready to go to ministers.
“It also contains a significant number of caveats and is hugely dependent on a wide range of assumptions which demonstrate that significantly more work needs to be carried out to make use of this analysis and draw out conclusions,” the source said.