Independent Group MPs will bring contempt proceedings if the government fails to publish cabinet papers on the damage from a no-deal Brexit within 48 hours.

Both Chuka Umunna, a former Labour MP, and Anna Soubry, an-ex Conservative, warned Theresa May that the deadline – agreed in parliament two weeks ago – had to be met.

“If they don’t do so, we will immediately move to institute contempt proceedings against them in the Commons for going back on their promises,” Mr Umunna said.

Ms Soubry added: “This minister, with the agreement of the government, said, ‘We will give you these papers.’ These papers are really, really important.”

The threat follows Ms Soubry’s decision to delay a showdown on extracting the papers when the prime minister suffered her last Brexit defeat a fortnight ago, but only because of the promise that they would be published.

Britain Before Brexit: Northern Ireland Show all 12 1 /12 Britain Before Brexit: Northern Ireland Britain Before Brexit: Northern Ireland Derry, Londonderry A garage door displaying unionism, bolted shut, like a visual representation of Brexit Britain, locked to outsiders, safeguarding what’s inside Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Derry, Londonderry Rossville Street, the site of Bloody Sunday, where messages demand a severance with England. From this perspective, Britain is England in sheep’s clothing, the real empire, the centre of colonial power Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Bangor A political message in paint not yet dry, still forming, setting, adjusting, or in old paint finally eroding, melting away Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Bangor Moral judgement frames a residential view. The message seeks to make everybody involved in the religious narrative: those who don’t believe are those most in debt Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Castlerock The beach is sparse and almost empty, but covered in footprints. The shower is designed to wash off sand, and a mysterious border cuts a divide through the same sand Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Belfast Two attempts to affect and care for the body. One stimulated by vanity and social norms and narratives of beauty, the other by a need to keep warm in the winter night Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Belfast The gate to an unclaimed piece of land, where nothing is being built, where no project is in the making, where a sign demands the creation of something new Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Derry, Londonderry Under a motorway bridge a woman’s face stares, auburn and red-lipped, her skin tattooed with support for the IRA and a message of hostility to advocates of the Social Investment Fund Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Derry, Londonderry The Fountain Murals, where the curbs and the lampposts are painted the red, white, and blue of the Union Flag. A boy walks past in the same colours, fitting the scene, camouflaged Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Coleraine A public slandering by the football fields, for all to see or ignore. I wonder if it’s for the police or for the community Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Belfast A tattoo parlour, where the artist has downed tools, momentarily, bringing poise to the scene, which looks like a place of mourning, not a site of creation Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Derry, Londonderry A barrier of grey protects the contents of this shop, guarding it from the streets outside, but it cannot conceal it completely, and the colours of lust and desire and temptation cut through Richard Morgan/The Independent

They are expected to show a huge economic hit from crashing out of the EU without an agreement, which some economists believe could match the recession after the 2008 financial crash.

A previous government analysis forecast that GDP would slump by 9.3 per cent over 15 years, and all Brexit scenarios would leave Britain poorer.

Mr Umunna, writing in The Independent, said: “Two weeks ago the Independent Group MPs had tabled an amendment (in the name of Anna Soubry) to force ministers to [publish the papers] but we withdrew it when they relented and promised to publish these papers before the vote this week.”

Ms Soubry, speaking on the BBC’s Victoria Derbyshire show said: “What they show is an impartial, honest appraisal of the grave dangers to our country in trade and economic terms if we leave without a deal. We believe that the public have a right to see those papers.”

Meanwhile, Mr Umunna acknowledged that he disagreed with Ms Soubry over the austerity policies pursued by George Osborne as Tory chancellor.

But, speaking on the same programme, he denied that represented a problem for the fledgling group, saying: “This is what makes our group different. Shared progressive values and, of course, we are going to have taken different positions over history.”

“Our country is divided and if we are capable – coming from different political traditions – of reaching consensus on policy, then we have got a much better chance of uniting the country than these old tribal parties where they often oppose for the sake of it.”

Mr Umunna also said the Independent Group had received tens of thousands of pounds in small donations since it was founded last week, but was unable to give an exact figure.

He confirmed that Sir David Garrard, a former major Labour donor, had made a donation but that he did not know how large it was.

Asked whether taking money from wealthy donors indicated that the new group was getting involved in “old politics”, Mr Umunna said: “There’s nothing wrong with being a rich man and there’s nothing wrong with having a social conscience.”