The UK faces a "constitutional crisis" if Theresa May does not publish the full legal advice on her Brexit deal on Monday, Labour has warned.
The PM says the advice is confidential, but some MPs think ministers do not want to admit it says the UK could be indefinitely tied to EU customs rules.
Sam Gyimah, who quit the government on Friday, said releasing the advice was "key to restoring trust in politics".
Attorney General Geoffrey Cox will make a statement about it later.
He is set to publish a reduced version of the legal advice - despite calls from MPs from all parties to publish a full version.
MPs say this will not not respect a binding Commons vote last month, which required the government to lay before Parliament "any legal advice in full".
His statement to the House of Commons will be followed by five days of debate on the proposed Brexit deal.
Home Secretary Sajid Javid has dismissed speculation that the final vote on Mrs May's deal - due on 11 December - could be delayed, saying he didn't think there was "any chance" of that.
The BBC's deputy political editor John Pienaar said that, as it stood, it looked like Mrs May was facing a "heavy defeat" and, should this happen, no-one could be sure what might follow.
Legal issues to the fore
Ahead of the five days of debate on the deal, which begin on Tuesday, Labour is planning to join forces with other parties, including the DUP, which keeps Mrs May in power, to initiate contempt of Parliament proceedings unless the government backs down on publishing the full legal advice.
Shadow Brexit secretary Sir Keir Starmer told Sky News: "If they don't produce the advice... this will be a collision course between the government and Parliament."
His shadow cabinet colleague Barry Gardiner told the BBC's Andrew Marr show the prime minister faced a "very serious constitutional crisis" if she refused, and the only answer was a general election.
The former Lord Chancellor Lord Falconer told BBC Radio 4's Today programme that the public "have the full right to know" the legal advice and the debate in the Commons should "properly be informed" by it.
Mr Gyimah said the government had a duty to "level with the public".
But former solicitor general and Tory peer Lord Garnier told the programme it was a "matter of convention" that the advice was not disclosed, adding that it was government policy under attack and not the legal advice.
According to the Sunday Times, Mr Cox told ministers in a letter the only way out of the Northern Irish "backstop" agreement would be to sign a new trade deal, a process which could take years.
Brexit-supporting MPs say it could mean an open-ended commitment for the UK, forcing it to remain in the EU's customs union while details of the deal are being worked out.
Downing Street has not responded to
claims by the Daily Telegraph that a letter from Mrs May's chief Brexit adviser, Olly Robbins, warned her it was a "bad outcome" with no legal "guarantee" Britain would be able to exit the mechanism.
Boris Johnson, who resigned from government over the PM's Brexit vision,
described the arrangement in his weekly Daily Telegraph column as "a great steel trap that is about to clamp its jaws around our hind limbs and prevent our escape".
The DUP's Sammy Wilson said it was important for MPs to know exactly what they are voting for and the implications for Northern Ireland.
'Fight of her life'
The Commons debate on Theresa May's deal is due to get under way on Tuesday, with the prime minister facing the fight of her political life to get MPs to back the deal in a vote on 11 December.
Ministers insist they can persuade enough of them to change their mind because, they claim, the alternative is a no-deal Brexit or "no Brexit at all".
Mr Javid told BBC Radio 4's Today the agreement on the terms of the UK's exit and its future relations with the EU was "never going to be perfect" but it was the "right deal for the country".
The agreement, he insisted, delivered on the 2016 referendum result by bringing "a complete end" to EU free movement in the UK and replacing it with a skills-based immigration regime that would reduce inward migration to more "sustainable" levels while "meeting the demands of British industry".
But he conceded it was "very unlikely" MPs would see the details of what the UK's post-Brexit immigration system might look like before they vote, as the plans were still be worked through and people being consulted.
Meanwhile, as campaigners continue to push for a further referendum, an e-petition urging the government to rule that option out, will be debated by MPs from 16:30 GMT.
brick, meet fan