Prime Minister Theresa May has dismissed the significance of leaked Government studies which indicated that any outcome from Brexit would leave Britain worse off.
And Mrs May pledged that MPs will be given full and "appropriate" analysis papers on the likely impact of the final Brexit deal before they are asked to vote on it.
Papers prepared by the Department for Exiting the EU and obtained by website Buzzfeed suggested that even with a comprehensive trade deal of the kind Mrs May is seeking, UK growth would be down by 5% over the next 15 years.
This would rise to 8% if Britain left without a deal and was forced to fall back on World Trade Organisation rules.
On Tuesday one of Mrs May's ministers warned official assessments which predict an economic hit from Brexit cannot be dismissed.
But Mrs May said the findings were "very preliminary" and did not address the actual deal which she is hoping to conclude.
"It would be wrong to describe this as 'the Brexit impact assessment'," the Prime Minister told reporters travelling with her on a trade mission to China.
"There is analysis being done. This is very preliminary.
"What has been seen so far is a selective interpretation of a very preliminary analysis, which ministers have not signed off, have not approved, and which doesn't actually even look at the sort of deal that we want to deliver in terms of the future relationship with the European Union."
Mrs May has promised MPs a vote on the final deal reached with the EU.
And she said: "When the time comes for Parliament to vote on the final deal, we will ensure that Parliament has the appropriate analysis on which to be fully informed, on which to base their judgement.
"But it would be wrong to publish analysis before that analysis has been completed, and it would also be wrong to publish analysis which might prejudice our negotiating position, and indeed Parliament itself has accepted that."
Brexit-backing minister Steve Baker told MPs on Tuesday that official forecasts drawn up by civil servants were "always wrong".
However Justice Minister Philip Lee said if the figures in the leaked assessment are "anywhere near right" there should be a "serious question" about the Government's approach to Brexit.
He said: "The next phase of Brexit has to be all about the evidence.
"We can't just dismiss this and move on. If there is evidence to the contrary, we need to see and consider that too.
"But if these figures turn out to be anywhere near right, there would be a serious question over whether a government could legitimately lead a country along a path that the evidence and rational consideration indicate would be damaging.
"This shows the PM's challenge... The PM has been dealt some tough cards and I support her mission to make the best of them."
Labour is to seek to force the Government to release its latest assessment of the impact of Brexit on the economy through a binding Commons vote.
In an opposition day debate on Wednesday, the party will use the same archaic parliamentary procedure it adopted last year to force ministers to release Brexit impact papers to a Commons select committee.
The latest leaked study, drawn up for the Department for Exiting the EU, concluded the UK economy would lose out, whatever Brexit deal the Government struck with the EU.
Shadow Brexit secretary Sir Keir Starmer said MPs were entitled to the information if they were to make informed decisions on Britain's future relationship with the EU.
"People voted to leave the European Union in part to give Parliament control about its own future. That means giving MPs the information they need to scrutinise the Government's approach to Brexit," he said.
"Ministers cannot keep side-lining Parliament to hide the deep divisions within their own party.
"They should accept this motion and allow the country to have an informed debate about its relationship with Europe after Brexit."
Unlike a normal opposition day motion, the vote on the "humble address" will be binding on the Government if it is passed in Wednesday's debate.
In the Commons on Tuesday, a number of pro-Remain Tories joined opposition MPs in calling for the analysis to be released, suggesting the vote could be close.
Former chancellor Kenneth Clarke accused ministers of trying to protect the Government from "political embarrassment" in refusing to release the document.