Britain’s main Opposition 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.

The threat is yet another hurdle May must clear before parliament votes on December 11 on her deal for Britain’s exit from the European Union, its biggest shift in foreign and trade policy for over 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 with a 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.

The government has promised to give lawmakers 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 (Monday) 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.”