LONDON (Reuters) - The Labour Party said on Sunday it would press for contempt proceedings against the government if Prime Minister Theresa May fails to produce the full legal advice she has received on her Brexit deal.

Britain's Prime Minister Theresa May addresses the media during the G20 Leaders Summit in Buenos Aires, Argentina December 1, 2018. REUTERS/Carlos Garcia Rawlins

The threat is yet another hurdle for May to clear before parliament votes on Dec. 11 on her deal for Britain’s exit from the European Union, its biggest shift in foreign and trade policy for more than 40 years.

With the odds looking stacked against her, May is touring the country and media studios to try to win over critics including both eurosceptics and europhiles who say the deal will leave Britain a diminished state, still linked economically to the EU but no longer having any say over the rules.

May often says her deal will protect jobs and end free movement. She hopes her argument that it is the only feasible deal with the EU and that voting it down will raise the risks of a “no-deal” Brexit or no Brexit at all will concentrate minds.

Labour has said it will vote against the deal. On Sunday its Brexit spokesman, Keir Starmer, increased the pressure on May by saying Labour would start contempt proceedings against the government if it did not publish its legal advice.

He also said Labour would seek a vote of no confidence in the government if she lost the vote, a widely forecast outcome.

“In nine days’ time, parliament has got to take probably the most important decision it has taken for a generation and it’s obviously important that we know the full legal implications of what the prime minister wants us to sign up to,” Starmer said.

“I don’t want to go down this path ... (but) if they don’t produce it tomorrow then we will start contempt proceedings. This would be a collision course between the government and parliament,” he told Sky News.

British media said the contempt move was also supported by the small Northern Irish party which props up May’s minority government, underlining her precarious position in parliament.

“GIANT STEEL TRAP”

May’s former foreign secretary, Boris Johnson, a prominent opponent of May’s deal and advocate of a much sharper break with the EU, also weighed in.

In his weekly column in the Telegraph newspaper, he said he backed the calls for the government to publish the advice, adding that it showed what some feared - a so-called backstop arrangement for Northern Ireland was “a great steel trap that is about to clamp its jaws around our hind limbs and prevent our escape”.

The government has promised to give MPs access to the legal analysis of the Brexit deal and Attorney General Geoffrey Cox will make a statement to parliament on Monday. Opposition parties suspect it will only offer a summary of that advice.

“This is an unprecedented situation and that’s why we’ve got an unprecedented situation just tomorrow when the attorney general will be making a statement to parliament,” Conservative Party Chairman Brandon Lewis told Sky News.

“And I would hope again that when colleagues hear what the attorney general has to say, they will be satisfied that the government has delivered on what it said it would do.”

Under parliamentary rules, it is up to the speaker to decide whether to allow a contempt motion to be voted upon. If it passes, it would then be referred to a committee which would rule on whether contempt had taken place. If so, it would then recommend a punishment, which MPs must agree.

Critics of May say the advice could contain warnings about certain parts of her withdrawal deal with Brussels, especially over the status of Northern Ireland and, if published, might stiffen opposition to the accord.

But her environment minister, Michael Gove, again said that, while not perfect, the deal was the best Britain could get.

“I believe that we can win the argument and win the vote. I know it’s challenging but my view is ... that we’ve got to make those arguments and we’ve got to look properly at what those alternatives are,” he told the BBC’s Andrew Marr Show.