The PM’s decision to suspend Parliament for five weeks was carried out “in bad faith”, the Supreme Court has heard.
Scottish judges ruled the move unlawful, but the government is appealing, arguing it is a political judgement, not a matter for the courts.
Defending the ruling, Aidan O’Neill QC said people expected the government to engage “solely in high politics rather than low, dishonest, dirty tricks”.
But, he added: “I am not sure we can assume this of this government.”
The PM prorogued Parliament earlier this month, saying it would allow him to hold a Queen’s Speech on 14 October to outline his new policies.
He has insisted the decision had nothing to do with his promise to leave the EU on 31 October, with or without a Brexit deal, but critics have accused him of trying to silence MPs in the run-up to the deadline.
Two cases about the prorogation are now being appealed at the Supreme Court after lower courts reached conflicting judgements.
Businesswoman Gina Miller and other campaigners are appealing against a ruling by England’s High Court, which said the suspension was “purely political” and therefore “not a matter” for the judiciary.
The government, meanwhile, is appealing the ruling by judges at Edinburgh’s Court of Session, who said the move by Mr Johnson was “unlawful” and aimed to “stymie” MPs ahead of the Brexit deadline.
A senior government source told the BBC’s political editor No 10 believed the Supreme Court would judge that prorogation was a matter for the courts and would “fire warning shots about how a government should not use this to close Parliament illegitimately”.
However, Laura Kuenssberg said according to the source, No 10 did not believe the court would unravel their plan for a 14 October Queen’s Speech.
Speaking on behalf of the Scottish challengers to prorogation, Mr O’Neill said one of the advantages of the ruling from Edinburgh was it had “distance”, which lends “perspective”.
“A view of what all of this heated debate [and] political machinations looks like 400 miles away, far from the fever and excitement of the nation’s capital, [and] outside the Westminster bubble,” he said.
Mr O’Neill said he was not calling for the court to “decide a whole new set of rules for how prorogation can be used”, but it was “most certainly for the province of the courts” to determine whether the move followed constitutional principles.
He added: “In the present case, it appears the prime minister’s action… has had the intent and effect of preventing Parliament… from holding the government politically to account at a time when the government is taking decisions that will have constitutional and irreversible impact on our country.
“That cannot be a lawful use of the power of prorogation.”
He added: “We say it was for an improper purpose and done in bad faith.”
The QC suggested the government should engage “solely in high politics rather than low, dishonest, dirty tricks”, but “given the attitude that has been taken by its advisers and the prime minister to the notion of the rule of law” that could not be assumed.
He added: “We cannot have a situation in which there are no standards, in which prorogation can be used with impunity.”
Mr O’Neill said the Supreme Court’s own logo – which contains a symbol for each of the UK’s four jurisdictions – showed the rule of law in Scotland must be “respected” along with the others.
During the morning session, the Supreme Court’s 11 judges heard arguments from the government’s representative, Sir James Eadie QC.
He told the court prorogation was “a well-established constitutional function exercised by the executive” and decisions about it were “squarely… within that political or high policy area”.
“Such decisions are inherently and fundamentally political in nature,” he continued, accepting that those decisions were “inevitably shot through with assessments of a political kind”, including how a government might secure its “political objectives”.
Sir James argued Parliament had previously passed laws addressing aspects of prorogation, but there was no law relevant to this particular case.
Therefore, he said, the courts could not intervene in the decision.
When asked about the need to uphold parliamentary sovereignty, Sir James said it was “a precious principle”, but urged caution before “that phrase is too widely or generally bandied about”.
Prorogation is carried out by the Queen on the prime minister’s advice.
Lord Sales, one of the judges, asked whether “if there are constitutional principles that are required to be policed”, it would be more appropriate for the courts to do that “rather than for the Queen to be sucked in”.
In reply, Sir James said “constitutional protections are provided in the political arena”, via measures like no-confidence motions that could unseat a prime minister.
“That’s where you find the appropriate form of control, not the courts.”
Tough questions for the government
Sir James’ submission was shot through with the idea that prorogation is not a matter for the courts.
But there were a couple of really interesting interventions from the judges.
Lord Wilson asked why there were no witness statements setting out reasons for prorogation.
That seemed to take Sir James aback – you have the documents you have, he said.
Lord Kerr then asked if the five week prorogation gave the government a political advantage.
He said it did, but that it was not unlawful.
So two tough questions for the government.
One of the judges, Lady Black, said the government’s submission was underpinned by the argument that the courts could not get involved because Parliament already acted as a check on government.
But, she asked, how could Parliament apply a check on government once Parliament is “removed from the picture”?
Sir James said prorogation always and inevitably had the effect of limiting debate in Parliament, but MPs could resume their scrutiny once the suspension was over.
It was not appropriate for the courts to “design a set of rules” about how long a suspension should last, he said.
He also pointed out the prorogation dates had not stopped MPs being able to pass a law blocking a no-deal Brexit.
Lord Wilson questioned why no witness statement had been provided by a minister to explain why the prorogation decision was taken.
He said: “Isn’t it odd that nobody has signed a witness statement to say ‘This is true, these are the true reasons for what was done’?”
Just before the morning session concluded, Sir James told the court he would present a written statement setting out what the government would do if it lost the case.
On Tuesday, Advocate General for Scotland, Lord Keen QC, on behalf of the government, assured the court the prime minister would take “all necessary steps” to comply in that event, but would not comment on whether Mr Johnson might subsequently try to prorogue Parliament again.