Pressure is growing on Theresa May to publish the Government's latest Brexit analysis after a report that 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.
Government sources played down the leak, saying the document was an "early draft" and that "significantly more work" was needed before the analysis was complete.
However Labour shadow Brexit minister Matthew Pennycook said: "Labour has made clear since the referendum that Tory ministers cannot withhold vital information from Parliament and the public about the impact of different Brexit scenarios on jobs and the economy.
"Ministers should publish this information immediately and allow for a full debate in Parliament about its implications."
Liberal Democrat spokesman Tom Brake said: "Theresa May has been trying to hide the truth from the public for months but now her mask is slipping.
"The cold hard reality is that these reports reveal nothing we didn't already know. Of course staying in the EU remains our best option, of course Brexit is going to damage the UK."
The analysis, drawn up for the Department for Exiting the EU and seen by the BuzzFeed News website, found economic growth would be lower under a range of potential scenarios.
Even if the UK is able to negotiate a comprehensive free trade agreement, as the Government hopes, it estimated growth would be down 5% over the next 15 years.
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%.
The leak did nothing to calm the febrile mood within the Conservative Party, amid concerns over Mrs May's leadership and the direction of the Brexit negotiations with Brussels.
Former Cabinet minister Iain Duncan Smith said the leak was "highly suspicious" and previous forecasts about the impact of Brexit had proved to be "completely wrong".
"I think 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.
He told the BBC: "I think we should take this with a pinch of salt."
A Government source said the analysis had studied the impact of a series of "off-the-shelf" agreements but had not considered Mrs May's preferred option of a "bespoke" agreement.
"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 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."