If the UK government fails to release its full legal advice on the Brexit deal, Prime Minister Theresa May will most likely face a "historic constitutional row that puts Parliament in direct conflict with the executive", according to the Labour Party's Brexit spokesman, Sir Keir Starmer.
He pointed out that "all parties" would press for contempt of Parliament proceedings if MPs are not shown the advice, the legal opinion of British Attorney General Geoffrey Cox.
Starmer said that if May loses a Commons vote on her Brexit deal, scheduled for December 11, "then there has to be a question of confidence in the government."
The Democratic Unionist Party, in turn, signalled its readiness to join Labour and other opposition parties on Monday to write a joint letter to John Bercow, the House of Commons Speaker, unless cabinet ministers back down.
The Telegraph cited a draft of the letter as saying that the MPs believe that a government statement "does not constitute the final and full advice provided by the Attorney General to the Cabinet."
"It does not comply with a motion of the House that you have ruled to be effective. We would now ask that you consider giving the House of Commons the opportunity to debate and consider this matter of contempt at the earliest opportunity," the letter's text reads.
The spat over the legal advice came after the government insisted on publishing only "a reasoned position statement”, rather than the full advice, in line with the parliament's demands.
Earlier, Attorney General Cox warned in a letter to the cabinet ministers that the UK could be tied to the EU customs union "indefinitely" by means of the Northern Ireland "backstop" and that the only way out is to sign a new trade deal.
Speaking at the G20 summit in Argentina, Prime Minister May, for her part, tried to reassure G20 leaders that the Brexit deal would be of importance to the global economy.
"The next nine days are a really important time for our country, leading up to the [December 11] vote on this deal. I will be talking with members of Parliament obviously and explaining to them why I believe this is a good deal for the UK," she emphasized.
The deal's major controversies include the so-called "divorce bill", almost 40 billion pounds in aid (just over $50 billion) the UK has to pay the EU; another bone of contention is the unclear situation with the UK-EU border in Northern Ireland.