A judge has agreed to a fast-tracked hearing about whether Boris Johnson can legally suspend parliament to force through a no-deal Brexit.

A group of more than 70 MPs and peers is calling 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 – which include MPs Jo Swinson (Liberal Democrats), Heidi Allen (Change UK) and Stella Creasy (Labour) – filed a petition at the Court of Session in Edinburgh, in an attempt to block the prime minister from being able to prorogue parliament.

At a preliminary hearing on Monday, Lord Raymond Doherty agreed to expedite the timetable for the legal challenge to take place, setting the date for the substantive hearing for Friday September 6.

Doherty rejected the campaigners’ attempts to have the whole hearing before one court to speed up the process.

“It’s a case that requires expedition but there is a requirement for fairness to both sides,” he said.

Doherty announced that, under the accelerated timetable, a substantive hearing would take place on September 6 – nearly eight weeks before the Halloween departure date – with a preliminary hearing on September 4, if required to rule on any points of legal contention between the two sides.

The lawyer working for the campaigners, David Welsh, argued the case needed to be heard “swiftly” because any judgment that came after the date the Prime Minister has committed to leaving the EU “would be rendered academic by the passing of time”.

“We are facing consequences of such extreme and severe nature that it’s important for the court to fulfil its role in ruling on the law and how it should be applied,” he said.

“There is ample evidence that the respondent will try to slow down as much as possible.”

Although acknowledging the “exceptional circumstances” of the case, Doherty told Welsh he would be refusing the request for the whole case to be heard by just one court.

“I’m not in a position to say that this is a case to report to the inner house,” he said.

“I don’t doubt your intentions but these are the practicalities.”

Andrew Webster, acting on behalf of the advocate general, rejected the suggestion the government wanted to delay proceedings to make any judgment irrelevant if not made by the October 31 deadline.

The financial liability in the case was also 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).

Webster pointed out that, among the signatories of the legal challenge, were almost 70 MPs with combined parliamentary salaries of approximately £5.5 million.

“Any pain is likely to be shared, as it were,” the judge declared.

But he said: “On balance I am persuaded – given that parliamentarians are bringing a matter, in their view, that is being raised as a matter of public service – that it is unfair to consider their personal assets when coming to a view on whether a PEO is made.”

The legal papers, submitted by the Good Law Project acting on behalf of the politicians, 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.