The government could be forced to allow MPs to see a secret analysis setting out the predicted negative economic impact of Brexit after Labour said it would use a vote to invoke an obscure piece of parliamentary procedure.
On Monday night, BuzzFeed News published details of the January 2018 analysis, which was produced by government economists for the Brexit department and was only intended to be seen by cabinet ministers and senior civil servants. It concluded the British economy will be worse off in every single post-Brexit scenario they modelled.
The leaked assessment, which also said that every region and almost every sector of the economy would be adversely hit under every scenario, unleashed chaos in the government on Tuesday.
As some pro-Remain senior Tories joined Labour in demanding the government publish the analysis and Brexiteer MPs dismissed it as incomplete and unreliable, Brexit minister Steve Baker was called to the House of Commons to answer two hours' worth of questions.
He used his time at the despatch box to rubbish the quality of his own government's economic projections, and insisted it would not be in the "national interest" to publish the full document, since it could undermine Britain's negotiations with the EU.
But on Tuesday night, shadow Brexit secretary Keir Starmer pledged to use an opposition day in Commons on Wednesday to hold a vote that could compel the government to release the document in full.
Speaking on the Today programme on Wednesday morning, Starmer said the government couldn't argue, as ministers and pro-Brexit MPs have, that the release of the document would hinder Brexit negotiations.
"There's obviously a difference between something that undermines negotiations and embarrasses the government," he said.
"What really weakens that line of defence for the government is that yesterday ... Steve Baker, a government minister, actually started using some of the figures to make his own argument, instead of saying 'look, these are confidential, we can't release them'. He actually used the figures that advantaged him and said he wouldn't release the others."
Labour intends to invoke the "humble address", an obscure piece of parliamentary procedure which has been rarely used since the 19th century. It can effectively compel the government to provide documents to parliament, by virtue of appealing directly to the Queen for access to information.
The same method was used in November by MPs to force the government to release the secret impact assessments Brexit secretary David Davis said had been carried out on 58 different sectors of the economy. In the end, Davis said the assessments did not exist and instead the government published a series of "sectoral analyses" instead.
"Once again Labour has been forced to use an archaic parliamentary process to make ministers do the right thing," said Starmer. "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."
Labour is likely to be victorious again, since the government has rarely voted against opposition day motions since the general election, although they are rarely binding decisions.
Even if the humble address went to a vote, it is probable that the document would be released because the Conservatives do not have an outright majority in the House of Commons and many Tory MPs have already called for it to be published in full.
The announcement came as more Conservatives raised concerns about the economic projections.
In a significant intervention on Tuesday night, Tory justice minister Phillip Lee became the first member of the government to say the leaked findings should not be dismissed.
In a series of tweets, Lee said "the next phase of Brexit has to be all about the evidence," and added: "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."
Lee, a prominent moderniser from the centrist wing of the party, concluded: "We must act for our country’s best interests, not ideology & populism, or history will judge us harshly. Our country deserves no less."