Ministers have sought to play down the leak of a government document predicting an economic hit from Brexit.
According to BuzzFeed, the report concluded growth would be lower in each of three different Brexit outcomes than if the UK had stayed in the EU.
Responding to an urgent question, Brexit minister Steve Baker told MPs the document was at a “preliminary” stage and “requires further work”.
But Labour said it should be published in full and debated in Parliament.
Eurosceptics said other forecasts had been wrong and this one should be taken “with a pinch of salt”.
According to Buzzfeed, the leaked document, titled EU Exit Analysis – Cross Whitehall Briefing and drawn up for the Department for Exiting the EU, suggests almost every part of the economy would suffer.
It looked at scenarios ranging from leaving with no deal to remaining within the EU single market.
Mr Baker said the document was “not anywhere near being approved by ministers” and that ministers in his team had only just seen it.
He described the document as a “preliminary attempt to improve on the flawed analysis around the EU referendum” and said it did not asses the government’s preferred option of a bespoke free trade deal.
Responding to calls for it to be published, he said: “We don’t propose to go into these negotiations having revealed all of our thinking.”
And as Brexiteer MPs hit out at the leaking of the document, he said there was “clearly” a campaign to overturn the 2016 EU referendum by some people in the media and the House of Commons.
Responding for Labour, Brexit spokesman Sir Keir Starmer said the emergency of the report was “piling absurdity on absurdity” with the government having previously denied the existence of Brexit impact assessments.
The SNP called for continued single market membership, saying the “highly embarrassing” leak was “not surprising”.
The BBC understands the Treasury contributed to the document but sources say it is part of a much wider range of work going on in Whitehall.
The report suggests UK economic growth would be 8% lower than current forecasts, in 15 years’ time, if the country left the bloc with no deal and reverted to World Trade Organisation rules.
It says growth would be 5% lower if Britain negotiated a free trade deal and 2% lower even if the UK were to continue to adhere to the rules of the single market.
All scenarios assume a new deal with the US.
Earlier Conservative MP and leave campaigner Iain Duncan Smith told BBC Radio 4’s Today programme the document should be taken “with a pinch of salt” as almost every single forecast on Brexit has been wrong.
“It’s an incomplete report… deliberately leaked because it gives a bad view,” he said.
A government source said: “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.
“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.”
Meanwhile, International Trade Secretary Liam Fox has denied telling Tory Brexiteers to prepare for disappointment.
Speaking to The Sun about pressure on Prime Minister Theresa May, he criticised Tory MPs involved with negative briefings, saying “nothing that would happen would change the parliamentary arithmetic”.
“We don’t have a working majority, other than with the support of the Democratic Unionists, and we need to accept the reality of that. I know that there are always disappointed individuals but they’re going to have to live with disappointment.”
Mr Fox told the BBC his warning was to Mrs May’s critics that they will be “disappointed” in their efforts to topple her or secure cabinet positions for themselves.