Theresa May is facing fresh pressure over a leaked Brexit analysis that concludes Britain would be worse off, whatever future deal is struck with the European Union.

Opposition MPs jumped on the revelations, demanding the public were told the true cost of leaving the EU.

May travels to China on Wednesday, leaving behind a Conservative Party in turmoil, riven by unrest among MPs over the direction of talks with Brussels.

The leak of latest economic impact assessment will do little to lighten their mood. Drawn up for the Department for Exiting the EU, the document shows 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.

However, 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, dismissed the findings as "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."

The disclosure comes amid anger among pro-Brexit Tories at the latest negotiating guidelines from Brussels which said the UK would remain subject to EU law - including any changes passed after it leaves in March 2019 - during a proposed 21-month post Brexit transition.

Amid fears among some Brexiteers that the Government is heading for a "soft" break, retaining many of the current elements of Britain's relationship with the EU, Rees-Mogg said the Prime Minister needed to spell out what sort of deal she was looking for.

"We would take for the first time since 1066, laws imposed on us by a foreign power without having a say over it ourselves. That may be acceptable if we have a clear idea what the end-point is," he told BBC2's Newsnight.