Parliament should be allowed to reassemble next week, the supreme court has been urged, as the legal battle over Boris Johnson’s five-week suspension threatened to escalate into a constitutional crisis over who has authority to recall MPs and peers.

At the end of the third and final day of an emergency hearing over the lawfulness of the prime minister’s advice to the Queen to suspend debates, the 11 justices were asked to encourage the Speakers of the Commons and Lords to reconvene the parliamentary session.

In his closing submissions, Lord Pannick QC, representing the legal campaigner and businesswoman Gina Miller, said that if the supreme court found that Johnson had acted unlawfully but he declined to end the suspension of parliament then “in those circumstances we believe it would be open to the Speaker and Lord Speaker to reassemble parliament … as soon as possible next week”.

But government lawyers delivered a defiant legal note to the court in which they asserted that the prime minister could legitimately suspend parliament again even if the supreme court ruled that the prorogation had been unlawful. It stated: “Depending on the court’s reasoning it would still either be open or not open to the prime minister to consider a further prorogation.”

Lord Keen QC, representing the government, cautioned the justices that the English and Scottish applicants in the appeal “were inviting the court into forbidden territory”. The decision to prorogue was political and not open to challenge in the courts, he declared.

The president of the supreme court, Lady Hale, promised that the court would deliver its ruling early next week. She added: “I must repeat this case is not about when and on what terms the UK leaves the EU. We are solely concerned with the lawfulness [of prorogation].”

The final afternoon saw a rapid sequence of exchanges between the justices, Keen and Pannick that exposed the lack of consensus over a pivotal process in the UK’s unwritten constitution – particularly, who has the power to summon parliament back.

In his closing submissions, Keen said the judges should decline to become involved. “Whether it is dissolution or prorogation [of parliament], this is forbidden territory … It is a matter between the executive and parliament.

“The applicants and petitioners are inviting the court into forbidden territory and an ill-defined minefield that the courts are not properly equipped to deal with.”

The court should read the “authentic and contemporaneous documents” that had been prepared for the case, Keen said. He dismissed the “discourteous and incendiary language” deployed by the barrister for the Scottish applicants, Aidan O’Neill QC, who accused the government of lying.

Even if the supreme court found Johnson’s advice had been unlawful, Keen maintained, the court should then leave it to the executive and parliament to respond. That suggestion prompted one justice, Lord Hodge, to ask how parliament could respond if it was prorogued and “wouldn’t be there”.

Another justice, Lord Kerr, suggested that if the court found that Johnson’s advice to prorogue had been unlawful that would consequently mean the prorogation instructions were also unlawful. The Scottish appeal court has already ruled that the prorogation is “null and void”.

The question of what “relief” or “remedy” the courts should deliver if they find against the prime minister is complicated by the courts’ reluctance to be seen to be trespassing on what many consider to be parliamentary privileges.

Pannick told the justices: “We would expect, in the light of a declaration [of unlawfulness], that the prime minister ensures that parliament resumes as soon as possible next week … We would ask that the court give an indication if it thinks it appropriate what it expects next week so that parliament will meet to decide what happens next.”

He continued: “It may be that the [Commons] Speaker and Lord Speaker will take action to ensure that parliament reopens as soon as possible next week and then parliament can debate and will decide exactly how it wishes to proceed.”

Earlier Lord Garnier QC, a Conservative former solicitor general who intervened in the case on behalf of Sir John Major, accused Downing Street of releasing “misleading” statements about prorogation of parliament and publishing excuses for Johnson’s five-week suspension of the Commons that “are not the true reasons”.

The extraordinary clash of evidence from two Conservative prime ministers came as the court grappled with questions involving the trustworthiness of Johnson’s newly appointed administration.

The Downing Street press office, Garnier told the court, had been “misleading” in releasing announcements during the summer that initially suggested that reports of an extended prorogation were “entirely false”.

“Its effect was plainly to mislead,” Garnier added.

Supreme court prorogation hearing: who are the judges? Read more

As to the cabinet minutes, which were subsequently published, Garnier said, they “are not the true reasons”. The cabinet documents suggested that the need for a long suspension was to prepare for a Queen’s speech and a break for the conference season.

Q&A What is the UK supreme court and how does it work? Show Hide The supreme court of the United Kingdom was formally established on 1 October 2009. It is the ultimate court for hearing cases of the greatest public or constitutional importance, and is the final court of appeal in the UK, except for criminal cases heard in Scotland. The supreme court took over the judicial functions of the House of Lords, separating the judiciary from the legislature. It brought an end to the system of appealing to the "lords of appeal in ordinary" - commonly known as the law lords. The court comprises the president and deputy president and 10 other judges of the supreme court. Judges are appointed by the Queen on the advice of the prime minister, to whom a name is recommended by a special selection commission. The prime minister is obliged to recommend this name. The judgments in all decided cases from the court are published on the court's website.

In submissions to the high court earlier this month, Garnier obliquely compared Johnson to an estate agent, citing a 2009 New Zealand case of Premium Real Estate involving a salesman who misrepresented a buyer as a genuine purchaser when in fact they wanted to resell the property for a quick profit.

“It could hardly be suggested that the duties of the prime minister to the monarch are less than those of an estate agent to a homeowner,” Garnier noted. “Accordingly, if the court is satisfied that the prime minister’s decision was materially influenced by something other than the stated justification, that decision must be unlawful, irrespective of whether the unstated justification was itself legally impermissible.”

The 11 justices now begin a behind-the-scenes debate among themselves to decide on their judgment. The process traditionally starts with the latest recruit to the supreme court bench – in this case Lord Sales – opening discussions with their initial conclusions and views. The bench is likely to be divided by such crucial, constitutional issues. In its article 50 decision in 2017, the supreme court ruling was returned by an eight to three majority.