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 that the UK could be indefinitely tied to EU customs rules.

Former foreign secretary Boris Johnson has joined calls for its publication, which critics say could sink the PM’s deal.

Attorney General Geoffrey Cox will make a statement about it on Monday.

He is set to publish a reduced version of the legal advice – despite calls from MPs from all parties to publish a full version.

His statement to the House of Commons will be followed by five days of debate on the deal.

MPs say the statement from the attorney general does not respect a binding Commons vote last month, which required the government to lay before Parliament “any legal advice in full”.

Labour is planning to join forces with other parties, including the DUP, who keep Mrs May in power, to initiate contempt of Parliament proceedings unless the government backs down.

Shadow Brexit secretary Sir Keir Starmer told Sky News: “If they don’t produce [the advice] tomorrow (Monday) then we will start contempt proceedings. 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.

“Surely we must be transparent about the most important treaty of the last 40 years,” the Labour peer said.

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.

Mrs May faces the fight of her political life to get MPs to back the deal in a Commons vote on 11 December.