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 with staying in the EU.
A cross-party amendment outlining the demand was backed by more than 70 MPs from six political parties, including enough Conservative rebels to deprive the government of its majority.
Facing the prospect of a humiliating Commons defeat, ministers bowed to pressure and agreed to accept the motion.
It means that, before MPs vote on Ms May’s Brexit deal, the government will have to publish 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 information will be made available at least a week before MPs hold a “meaningful vote” on whether or not to accept Ms May’s Brexit deal.
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 outcome of Britain remaining in the EU.
Less than an hour before a scheduled vote on the amendment, Treasury minister Robert Jenrick confirmed the government would publish the full set of comparisons.
He told the Commons: “The government has already confirmed that before we bring forward the vote on the final deal, we will ensure that Parliament is presented with the appropriate analysis in good time to make an informed decision.
“I can confirm that this analysis will bring together evidence from across government, insight from external stakeholders, a range of data and analytical tools. The analysis will consider the long-term costs and benefits of moving to new trading relationships with the EU and the rest of the world.”
He added: “Having considered the amendment and spoken to a number of [MPs], I’m happy to confirm that the baseline for this comparison will be the status quo that is today’s institutional arrangements with the EU.
“The analysis will consider a modelled no-deal scenario on World Trade Organisation terms, a modelled analysis of a [free trade agreement] scenario and a modelled scenario of the government’s proposed deal, and in each case these will be compared against the status quo of the current institutional arrangements within the EU.”
However, he dismissed calls for the Office for Budget Responsibility to publish an independent assessment of the accuracy of the government’s analysis, saying this was beyond the watchdog’s remit and would set an “undesirable precent”.
Previous leaked documents suggest the government’s analysis 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 for a fresh referendum after Ms May admitted that Brexit could still be stopped. They will consider it a major victory that the prime minister’s proposed deal will be officially compared with the potential merits of staying in the EU and not merely considered against the risks of a no-deal Brexit.
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.
“Let parliament see the figures, and then MPs can weigh up just how bad the trajectory the country is on will be for our communities.”
Theresa May: ‘It’s the future relationship that delivers on the Brexit vote. It’s the future relationship that actually says this is the right deal for the future for our country’
Speaking after ministers agreed to back down, he added: “The government was planning to con the British people. It is vital at this crucial time for our country that MPs and the public know the full facts about the cost of Brexit and how it compares to the deal we already have inside the EU.”
The latest blow to the prime minister’s authority came as Tory Brexiteers scrambled to secure enough signatures to trigger a vote of no confidence in her.
Despite claiming they would have reached the threshold of 48 by Monday night, the rebels appeared to still be short of support as their attempt to oust Ms May rapidly lost momentum.
But Ms May faced a further headache as the DUP, whose votes she relies on in Parliament, broke their agreement to back the government on key issues and abstained during several Finance Bill votes.
Eight of the party’s ten MPs also backed a Labour amendment on child poverty.
It raises urgent questions over the future of the pact between the DUP and the Tories just weeks before Parliament is expected to hold the crunch vote on the prime minister’s Brexit plan.
Ms May will need the DUP’s support if her deal is to have any chance of passing the Commons.
The Independent has launched its #FinalSay campaign to demand that voters are given a voice on the final Brexit deal.