Eleven Conservatives – including Jo Johnson, who resigned as a minister last week – have publicly signed up to the cross-party push, with the rebellion set to grow if it comes to a vote on Monday.
The prime minister has so far refused to commit to releasing the analysis, which is likely to underline how remaining in the EU would deliver a more prosperous future for the country.
Ms May meanwhile continued her media offensive to defend her Brexit plans, while eurosceptics pushed to raise enough support to trigger a vote of no confidence in the Conservative leader.
On Monday more than 70 MPs from six different parties will attempt to push through an amendment to the Finance Bill, which if passed would obligate Ms May to publish an economic impact assessment into the withdrawal deal she has agreed with the EU.
The names include 11 Tories – the same number who rebelled the last time Ms May was defeated in the Commons on Brexit – who had backed Remain in 2016, many of whom now also support a new final say referendum on the outcome of Brexit.
Mr Johnson, who quit the government branding Ms May’s deal a “failure of British statecraft on a scale unseen since the Suez crisis”, is expected to make his first speech in the chamber since resigning in support of the move.
The ex-transport minister told The Independent: “It is of crucial importance that the full study is published showing the impact of the government’s proposed terms of departure compared with the benefits we currently enjoy as full members of the European Union.
“The government needs to look MPs and the public in the eye and be able to say that this deal will make us better off, give us greater control and fulfil all the promises that were made two years ago.
“The reality is they won’t give us the truth because they can’t say that we will be better off with this deal than we are at the moment. That’s why even prominent Brexiteers are saying we would be better off staying in the EU than leaving with this deal – an extraordinary state of affairs.”
Ex-chancellor Ken Clarke said publication is necessary so that MPs were not impeded in making a decision on the Brexit proposals being put to them.
He added: “The only way to prevent the current shambles from becoming a serious national crisis will be for parliament to exercise a well-informed and sensible judgement in the national interest when it votes on all the key issues.”
The assessment would have to include a comparison between the current terms of the UK’s EU membership and the proposed agreement set to be signed off at an EU summit in a week’s time.
It would also have to be made public before MPs vote on whether to accept Ms May’s Brexit plans and the independent Office for Budget Responsibility would have to evaluate its accuracy.
Mr Umunna said: “The prime minister told the House of Commons last week that the country faces three choices – no Brexit, no deal and her agreement – and 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.”
Ms May and other ministers have refused to specifically say they would publish such an analysis, and have instead stuck to the more vague line that MPs would be given the “appropriate information” before voting on Ms May’s Brexit plans.
However, an earlier analysis that was leaked suggested that over a 15-year period GDP growth would be 8 per cent lower under a no-deal scenario than if the UK stayed in the EU.
It would then be 5 per cent lower under a free trade agreement and about 2 per cent lower with a soft Brexit option of single market membership.
Downing Street would not comment on the amendment on Sunday, but is likely to set out its position if it is called in the chamber on Monday – something that is likely given the broad cross-party support.
Ms May will head to Brussels this week for a meeting with EU Commission president Jean-Claude Juncker to hammer out remaining areas of the outline agreement on the future relationship ahead of a summit next Sunday.
But she poured cold water on the idea that the other part of the proposed Brexit deal – the withdrawal agreement – can be reopened now that negotiators have settled on an agreed version.
There is a push from five key members of her cabinet, including Andrea Leadsom and Michael Gove, to renegotiate parts of the withdrawal agreement relating to the “backstop” – which sets out what happens to the border between Northern Ireland and the Republic if no Brexit deal is reached by December 2020.
But the prospects of reopening talks on the text ahead of the summit look slim, with Irish prime minister Leo Varadkar again ruling it out on Sunday.
Meanwhile, the chair of the Tory committee in charge of administering leadership contests revealed that the number of MPs’ letters calling for Ms May to resign had not reached the critical point to force a vote of no confidence.
The admission will be a blow for Steve Baker, Jacob Rees-Mogg and other members of the eurosceptic European Research Group, who argue Ms May’s deal does not deliver a proper Brexit and are trying to depose Ms May as a result.
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