Theresa May has been dealt a fresh blow after MPs forced the government to agree to publish analysis comparing the economic impact of her Brexit deal compared to staying in the EU.
The cross-party amendment was backed by more than 70 MPs from six political parties, including enough Conservative rebels to deprive Theresa May of her majority.
Facing a certain defeat in the Commons, ministers caved and agreed to accept the motion.
It means that, before MPs vote on Ms May's Brexit deal, the government will have to publish Treasury analysis comparing the impact of the proposed agreement with the consequences of the alternative scenarios of a no-deal Brexit and remaining in the EU.
The Office for Budget Responsibility would also have to publish an independent assessment of the accuracy of the government's analysis.
The information will have to be made available at least a week before MPs hold a "meaningful vote" on whether or not to accept Ms May's Brexit deal.
The amendment to the Finance Bill was tabled by Labour's Chuka Umunna and the Conservative former minister Anna Soubry, who have been at the forefront of attempts to force a fresh referendum on Brexit.
Tory backers of the motion included Jo Johnson, who resigned as a transport minister earlier this month in order to criticise Ms May's Brexit plan.
The government has previously promised to publish Treasury analysis of the impact of the prime minister's deal compared to a no-deal outcome, but it was not expected to release a comparison with the consequences of Britain remaining in the EU.
Less than an hour before a planned vote on the amendment, Treasury minister Robert Jenrick confirmed the government would publish the full set of comparisons.
Previous leaked documents suggest the assessment is likely to show that remaining in the EU would be better for the economy than Ms May's proposed deal.
Anti-Brexit MPs hope to ramp up the pressure after Ms May admitted that Brexit could still be stopped.
Mr Umunna said the amendment could "change everything".
Writing exclusively for The Independent, he said: "In a big admission – given she has been peddling, for many weeks, the falsehood that the choice is between her deal and no deal – the PM conceded in the House of Commons last week that the country actually faces three choices: no Brexit; no deal; or her agreement.
"Therefore it is only right that MPs are provided with an economic analysis with a comparison between those options. Anything less would amount to pulling the wool over the eyes of parliamentarians and the people we represent.
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