Labour could seek to force publication via Commons motion as MPs across political spectrum demand to see document

Pressure on the government to provide details of legal advice about a possible Brexit deal is growing, with the DUP, Labour and Lib Dems demanding it be published, following calls from cabinet ministers to see the full document.

One option could be for Labour to seek to force publication via a Commons motion, as the party did with the government’s Brexit impact assessments.

Jeffrey Donaldson, the DUP’s chief whip at Westminster, said the party, which supports Theresa May in government, would like to see the full document published, allowing not only ministers but MPs and the public to assess it.

The shadow Brexit secretary, Keir Starmer, said the advice must be released to MPs so they can scrutinise the document, while the Lib Dems called for the advice to be published in full.

The advice, drawn up by the attorney general, Geoffrey Cox, at the request of Theresa May, looks into the various options connected to the backstop, seen as the final major impasse before a deal can be agreed.

At a cabinet meeting on Tuesday, Cox gave ministers a summary of the advice, and told them that if the UK insisted on the right to unilaterally end a backstop, opposed by the EU, it increased the risk of no deal.

It is understood that some ministers, among them Michael Gove, asked the prime minister whether they could see the full legal advice drawn up by Cox, rather than just hearing his summary and interpretations.

Donaldson told BBC Radio 4’s Today programme: “It’s in the public interest that we understand fully what is happening here. We’ve had that commitment already from the government, that they will tell us what the legal advice they have is in relation to the backstop.”

He said: “If the House of Commons is going to have a meaningful vote on a deal upon which this legal advice is very important, then I think people are entitled to know what that legal advice is.”

The Lib Dems called for the advice to be published. The party’s Brexit spokesman, Tom Brake, said: “Refusing to publish legal advice on Brexit makes a mockery of the discredited mantra ‘take back control’.”

Speaking in Brussels, where he is meeting EU leaders, Starmer said a backstop agreement had to be robust, meaning it was “essential MPs are given the opportunity to scrutinise the attorney general’s legal advice before voting on the final deal”.

He said: “The public have the right to know precisely what the cabinet has signed up to and what the implications are for the future.”

Labour’s aim would be for the advice to be available to MPs, as happened with the Brexit impact assessments. They were made available after Labour forced their release through a so-called humble address motion.

This, or an amendment to a bill, remain options for extracting the legal advice, but it is understood Labour want to first await the government’s response.

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Downing Street has said there is a long-standing convention that the government does not discuss legal advice, “or the existence thereof”.

Under the backstop idea, which officials will try to finalise before another potential cabinet meeting later in the week, the UK could maintain a temporary customs arrangement with the EU without being forced to accept a border in the Irish Sea.

May told her cabinet to “stand by their diaries”, with government sources suggesting ministers could be summoned for an emergency meeting later this week to sign off the backstop proposal before presenting it to Brussels, possibly later this month.

She warned that, while she wanted to strike a deal, this could not be “at any cost” and would depend on an “acceptable” framework for a future relationship with the EU, expected to be set out in a separate political declaration.

Hopes have grown in recent days that the issue of the backstop – which seeks to put in place a guarantee for the EU to avoid a hard Irish border if no permanent solution can be found – could be resolved, paving the way for a deal.

Donaldson reiterated that while his party hoped for an agreement, it believed a no-deal departure remained very possible.

“We haven’t got a deal at the moment, and it’s clear from the rhetoric coming from both Brussels and Dublin that they are so far opposing what the prime minister has suggested in terms of pragmatic arrangements to deal with the Irish border,” he said.

The party’s concerns about the backstop went beyond whether it would be time-limited, Donaldson said. He agreed that having no time limit was seen as common sense.

“If that was what the backstop was just about, yes, it would be,” he said. “But it’s not, of course, because the backstop is about effectively annexing Northern Ireland from Great Britain, in terms of tying it into a single market separate from the rest of the United Kingdom.”