The PM is “entitled” to suspend Parliament, even if it is for political reasons, the Supreme Court has heard.
A Scottish court ruled the five-week suspension was “unlawful”, but Lord Keen QC, for the government, said the decision was not one for the courts.
The Supreme Court is hearing two appeals relating to the decision by Boris Johnson to prorogue Parliament.
Lord Pannick QC, for campaigners against the move, said it was done to “silence” MPs in the run-up to Brexit.
He was representing an appeal, brought by businesswoman and campaigner Gina Miller, against a contradictory English High Court ruling that – unlike the judgement in Scotland – said the suspension was “purely political” and therefore “not a matter” for the judiciary.
Mr Johnson maintains it was right and proper to terminate the last session of Parliament to pave the way for a Queen’s Speech to outline the government’s legislative plans for the year ahead.
He insisted the move had nothing to do with Brexit and his “do or die” pledge to take the UK out of the EU on 31 October, with or without a deal.
Lady Hale, President of the Supreme Court, stressed the outcome of the two appeals would have no bearing on the timing of Brexit.
In her opening statement, the most senior judge in the UK said she and her 10 colleagues would endeavour to address the “serious and difficult questions of law”, but would not determine “wider political questions” relating to the Brexit process.
Last week, Edinburgh’s Court of Session found in favour of a cross-party group of politicians challenging the PM’s move, ruling the shutdown was unlawful and “of no effect”.
Scotland’s highest civil court found Mr Johnson had effectively misled the Queen in the sovereign’s exercise of prerogative powers.
SNP MP Joanna Cherry – who was also one of the lawyers on the case – told the BBC she was “cautiously optimistic” the Supreme Court would uphold the Scottish court’s ruling.
But, she added: “If they don’t, then they will be accepting that it’s possible under the British constitution for the prime minister of a minority government to shut down Parliament if it is getting in his way, and that just can’t be right.”
Responding for the government, the Advocate General for Scotland, Lord Keen, told the Supreme Court he took “no issue” with the court’s ruling that the decision could have been unlawful, but said to “declare that the prorogation is null and of no effect” was a step too far, adding: “They have simply gone where the court could not go.”
He said if the ruling was upheld, the prime minister would take “all necessary steps” to comply.
However, after being pushed by the judges, he said he would not comment on whether Mr Johnson might subsequently try to prorogue Parliament again.
Lord Keen said previous prorogations of Parliament had “clearly been employed” when governments wanted to “pursue a particular political objective”, adding: “They are entitled to do so.”
He said if MPs did not want Parliament to be suspended they had “adequate mechanisms” and opportunities to stop it in its tracks by passing new laws – pointing to the fact a bill to block a no-deal Brexit was passed in just two days.
Lord Keen added: “It is not for the courts to overlay their views on when Parliament should or should not have [acted]… that would be to intrude over the boundaries of the separation of powers.”
Appealing against the English court ruling, crossbench peer Lord Pannick told the Supreme Court Mr Johnson suspended Parliament to avoid the risk of MPs “frustrating or damaging” his Brexit plans, and there was “strong evidence” the PM saw MPs “as an obstacle” and wanted to “silence” them.
He said he had “no quarrel” with a prime minister’s right to prorogue Parliament in order to present a Queen’s Speech.
However, he said the “exceptional length” of this suspension was “strong evidence the prime minister’s motive was to silence Parliament because he sees Parliament as an obstacle”.
Lord Pannick said the facts showed the PM had advised the Queen to suspend Parliament for five weeks “because he wishes to avoid what he saw as the risk that Parliament, during that period, would take action to frustrate or damage the policies of his government”.
Lord Pannick said the effect of the suspension was to take Parliament “out of the game” at a pivotal moment in the UK’s history and he disagreed with the High Court’s judgement that the issue was outside the scope of the courts.
“The answer is either yes, or it is no, but it is an issue of law, and the rule of law demands the court answers it and not say ‘it is not for us and it is for the discretion of the prime minister’.”
Amid the protests outside the Supreme Court, and the calm but focused legal debate inside, the first day of this potentially monumental constitutional battle came down to two questions.
Do judges have the power to stop a prime minister proroguing Parliament? And, if so, did Boris Johnson act illegally and mislead the Queen?
Lord Pannick QC, for Gina Miller, hammered out attack after attack.
Where was the prime minister’s witness statement showing he had an honest reason for closing Parliament? How could a prime minister who is accountable to Parliament prevent it from holding his feet to the fire?
Lord Keen QC, the government’s top lawyer in Scotland, argued judges couldn’t even consider these questions.
There was a key moment of political intrigue when the justices wondered what Mr Johnson would do if he were to lose.
But his lawyer could not say whether he might simply ask the Queen to re-open Parliament – and then shut it down again.
Ms Miller is seeking a mandatory order which would effectively force the government to recall Parliament, BBC legal correspondent Clive Coleman said.
Opposition parties have called for Parliament to be recalled but at a cabinet meeting on Tuesday, Mr Johnson told ministers he was “confident” of the government’s arguments.
He told the BBC on Monday he had the “greatest respect for the judiciary”, and its independence was “one of the glories of the UK”.
Downing Street has refused to speculate on how the government might respond should they lose this court case.
Pressed this morning, the Justice Secretary, Robert Buckland, declined to say whether Parliament would be recalled, or indeed whether the prime minister might seek to suspend Parliament for a second time.
Mr Buckland said any decision would hinge on the precise wording of the court judgement.
Nevertheless, defeat would be a significant blow.
It would be the first time in modern history that a prime minister had been judged to have misled Parliament.
And if MPs were recalled, Mr Johnson would almost certainly face contempt of Parliament proceedings, accusations that he’d lied to the Queen, and pressure to reveal more details about his negotiating strategy and his planning for no deal.
Defeat in the Supreme Court would also make it much harder for the prime minister to defy MPs for a second time as he has threatened to do over their bill to block a no-deal Brexit.
Elsewhere on Tuesday, the prime minister has discussed Brexit in a phone call with German Chancellor Angela Merkel.
No 10 said afterwards: “The prime minister reiterated that the UK and the EU have agreed to accelerate efforts to reach a deal without the backstop which the UK Parliament could support, and that we would work with energy and determination to achieve this ahead of Brexit on 31 October.”