Theresa May will plead with MPs to back her Brexit deal as her Government was embroiled in a constitutional row with Parliament.
The Prime Minister will begin five days of Commons debate on her Brexit plan before a major test of her authority in a crunch vote on December 11.
Commons Speaker John Bercow said there had been an “arguable case that a contempt has been committed” and ruled MPs should debate the issue on Tuesday.
Mr Bercow made the comments in response to demands from Labour, the DUP and four other opposition parties, which had complained that the summary legal advice released by the Government on Monday did not comply with a Commons resolution passed in November.
Attorney General Geoffrey Cox was repeatedly challenged about the issue in the Commons on Monday but insisted it was in the public interest for the advice he gave to ministers to remain confidential.
“The House has at its disposal the means by which to enforce its will,” he acknowledged.
“It can bring a motion of contempt and seek to have that motion passed and seek to impose through the committee, or whichever way it is appropriately done, to impose a sanction. I fully accept that.
“I don’t set myself up contrary to the House, I simply say that I cannot compromise the public interest.”
Shadow chancellor John McDonnell said: “The contempt the Tories have shown Parliament and the people of this country demonstrates that they are not fit for the office they hold.”
Shadow Brexit secretary Sir Keir Starmer said: “We have given ministers numerous opportunities to comply with the order of Parliament and to release the Attorney General’s full and final legal advice on the Government’s Brexit deal. However, they have refused to do so.
“We have therefore been left with no option but to press ahead with contempt proceedings.”
He added: “Even at this eleventh hour, I would urge ministers to step back from the brink and to not go down in history as the first Government to be found in contempt.”
Liberal Democrat Brexit spokesman Tom Brake said: “MPs expect the publication of the full legal Brexit advice before the debates on the withdrawal deal begin.
“The Government must not be allowed to use this chaotic situation to take focus away from the mess they are making of Brexit.”
The cross-party motion tabled by Labour, the SNP, Lib Dems, DUP, Plaid Cymru and Green Party states that the House of Commons finds ministers in contempt for failing to comply with the resolution passed by MPs in November demanding the legal advice and “orders its immediate publication”.
Solicitor General Robert Buckland told BBC’s Newsnight it was a “complete diversion” and a “concocted parliamentary parlour game that should be stopped”.
But Tory Eurosceptic Jacob Rees-Mogg told the programme: “I would say publish and avoid being in contempt of the House of Commons, which is a very serious matter.”
The issue risks overshadowing Mrs May’s effort to win over MPs ahead of the showdown in a week’s time.
The Prime Minister faces widespread opposition from MPs across the Commons – including both the Leave and Remain wings of her own party – ahead of the vote.
Over the coming days she will deploy senior Cabinet ministers to make the case, with Chancellor Philip Hammond, Foreign Secretary Jeremy Hunt and Home Secretary Sajid Javid expected to appear at the despatch box.
Mrs May will say that to respect the 2016 referendum result would require “a Brexit that takes back control of our borders, laws and money” and “a Brexit that sets ourselves on course for a better future outside the EU, as a globally trading nation, in charge of our own destiny and seizing the opportunities of trade with some of the fastest-growing and most dynamic economies across the world”.
The Prime Minister will assert that her Brexit deal, thrashed out over months of negotiations in Brussels, delivers on her commitments to end free movement and the jurisdiction of the European Court of Justice.
It will also ultimately result in “a new free trade area with no tariffs, fees, quantitative restrictions or rules of origin checks – an unprecedented economic relationship that no other major economy has”.
She will tell MPs: “The British people want us to get on with a deal that honours the referendum and allows us to come together again as a country, whichever way we voted.
“This is the deal that delivers for the British people.”
Critics of the deal seized on Mr Cox’s comments on the backstop measure aimed at avoiding a hard border with Ireland if no alternatives are found.
The backstop is highly controversial, with Brexiteer MPs claiming it traps the UK into obeying rules set by Brussels without a say over them.
Mr Cox acknowledged neither the UK nor the EU had the right to terminate the backstop agreement unilaterally if it came into effect, even if talks on a future trade deal to replace it broke down.
“I make no bones about it – I would have preferred to have seen a unilateral right of termination in this backstop,” he told MPs.
But he added: “I’m prepared to lend my support to this agreement because I do not believe that we’re likely to be entrapped in it permanently.”
DUP Westminster leader Nigel Dodds called for the Government to “bin the backstop”.
Former Brexit secretary Dominic Raab suggested it could take a decade for the backstop measure to be struck down by the European Court of Justice even though it was only intended to be temporary.