A fast-tracked court hearing will decide whether the prime minister can legally suspend Parliament to force through a no-deal Brexit, after a judge in Scotland agreed to set a timetable.
More than 70 MPs and Peers have banded together to call on the Scottish courts to rule that suspending Parliament to allow the UK to leave without a deal would be "unlawful and unconstitutional".
The anti-Brexit campaigners filed a petition at the Court of Session in Edinburgh attempting to block from being able to prorogue Parliament, and called for the case to be heard before the October 31 Brexit deadline.
At a preliminary hearing, Judge Lord Raymond Doherty agreed to expedite the timetable for the legal challenge to take place, setting the date for the substantive hearing as Friday September 6.
As ITV News Political Correspondent Paul Brand says, that makes the first week back in Parliament, rather hectic.
The campaigners had requested for a Scottish court to hear the case because the equivalent in England does not sit over the summer.
Lord Doherty rejected the campaigners' attempts to have the whole hearing before one court to speed up the process, rather than the normal legal procedures.
"It's a case that requires expedition but there is a requirement for fairness to both sides," Lord Doherty said.
The financial liability in the case was set at £30,000 for each side, with the government's lawyer persuading the judge to raise it from the £5,000 the campaigners wanted under a protective expenses order (PEO).
A cross-party group of politicians backed the legal petition, supported by the Good Law Project, which won a victory at the European Court of Justice last year over whether the UK could unilaterally cancel Brexit by revoking Article 50.
Jolyon Maugham QC, director of the Good Law Project, said: “A man with no mandate seeks to cancel Parliament for fear it will stop him inflicting on an unwilling public an outcome they did not vote for and do not want.
“That’s certainly not democracy and I expect our courts to say it’s not the law.”
The legal papers state: “Seeking to use the power to prorogue Parliament to avoid further parliamentary participation in the withdrawal of the UK from the EU is both unlawful and unconstitutional.”
Warning that “the exercise of the power of prorogation would have irreversible legal, constitutional and practical implications for the United Kingdom”, the challenge calls for the court to declare that proroguing Parliament before October 31 would be both unconstitutional and unlawful by denying MPs and the Lords the chance to debate and approve the decision.