Theresa May is facing demands to publish the Government’s latest Brexit analysis following a report it had concluded Britain would be worse off, whatever deal is struck with Brussels.
As the Prime Minister prepared to fly out on a three-day trade mission to China, opposition MPs said 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.
The mood in the party will not have been helped by the leak of 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 is able to negotiate a comprehensive free trade agreement - as Theresa May hopes - it estimated growth would be down 5% over the next 15 years, according to the document seen by the BuzzFeed News website.
That would rise to 8% if Britain left without a deal and was forced to fall back on World Trade Organisation (WTO) 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 2%.
Jacob Rees-Mogg, the chairman of the pro-Brexit European Research Group (ERG) of Tory MPs, said the findings were “highly speculative”.
He said similar modelling carried out by the Treasury ahead of the Brexit referendum - predicting large scale job losses if there was a vote to leave - had been “comprehensively wrong”.
However Labour MP Chris Leslie, a member of the Open Britain group which campaigns against a “hard” Brexit, said ministers must now release the findings in full.
“No one voted to make themselves or their families worse off,” he said.
“The Government must now publish their analysis in full, so that MPs and the public can see for themselves the impact that Brexit will have and judge for themselves whether it is the right thing for our country.”
In response to the leak a Government source said officials from across Whitehall were undertaking “a wide range of ongoing analysis”.
“An early draft of this next stage of analysis has looked at different off-the-shelf arrangements that currently exist as well as other external estimates,” the source said.
“It does not, however, set out or measure the details of our desired outcome - a new deep and special partnership with the EU - or predict the conclusions of the negotiations.
“It also contains a significant number of caveats and is hugely dependant 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.”
SNP Foreign Affairs spokesperson Stephen Gethins MP said: “The leaked analysis paper is highly embarrassing for Theresa May, but for those of us working to ensure the UK stays in the single market and customs union, it is not surprising.
“This analysis looked at three of the most plausible Brexit scenarios based on existing EU arrangements and there is no scenario that does not leave the UK worse off.
“The UK government must listen to parties across Westminster, and the devolved administrations, so we can protect our economy, business sectors and labour market from the cliff-edge that we are currently headed towards.”