Boris Johnson sought to suspend Parliament to avoid the risk of MPs “frustrating or damaging” his Brexit plans, the Supreme Court has heard.
Lawyers for campaigners challenging the suspension said there was “strong evidence” the PM saw MPs “as an obstacle” and wanted to “silence” them.
The judges are hearing two challenges relating to the five-week prorogation.
Lady Hale, President of the Court, stressed the landmark case 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” raised by the case.
But she said the court would not determine “wider political questions” relating to the Brexit process and its ruling would have no bearing on “when and how the UK leaves the EU”.
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Over the next three days, the Supreme Court will consider two separate legal challenges over whether Boris Johnson acted lawfully in advising the Queen to prorogue Parliament.
Mr Johnson maintains it was right and proper to terminate the last session of Parliament in order to pave the way for a Queen’s Speech on 14 October, in which his new government will outline its 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, if necessary without a deal.
But 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’s actions were motivated by the “improper purpose of stymieing Parliament” from properly scrutinising the government’s Brexit plans in the run-up to a crucial summit of EU leaders on 17 October.
It found that the prime minister had effectively misled the Queen in the sovereign’s exercise of prerogative powers.
But, in a separate case, London’s High Court rejected a challenge brought by businesswoman and campaigner Gina Miller, ruling that the suspension of Parliament was a “purely political” move and was therefore “not a matter for the courts”.
The government is now appealing against the ruling in Scotland, while Ms Miller is appealing against the High Court judgement.
Lord Pannick, the crossbench peer and QC representing Ms Miller, told the Supreme Court on Tuesday 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”.
The facts, he said, 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”.
Drawing attention to what he said was the PM’s failure to provide a witness statement explaining the basis of his actions, Lord Pannick said the court had a “common law duty” to intervene if the executive had used its powers improperly.
He said the effect of the suspension was to take Parliament “out of the game” at a pivotal moment in the UK’s history and contravened the principle of parliamentary sovereignty
“The basic principle is that Parliament is supreme. The executive is answerable to Parliament…
“Ministers are constitutionally the junior partner – and the real issue in this case is whether the junior partner may lawfully remove the scrutiny of his activities by the senior partner.”
Eleven of the Supreme Court justices – the largest possible panel – are hearing legal arguments from the English and Scottish court cases on Tuesday.
The government’s lawyers will then respond on Wednesday.
BBC legal affairs correspondent Clive Coleman says it is only the second time 11 justices will sit – the first time this happened was in Ms Miller’s successful challenge as to whether the prime minister or Parliament should trigger Article 50 to start the process for leaving the EU.
They will determine whether prorogation is a matter for the courts, and if so, will go on to rule definitively on whether Mr Johnson’s true motive was to undermine MPs’ ability to legislate and respond to events as the country prepares to leave the EU, our correspondent added.
Opposition parties, meanwhile, have called for Parliament to be recalled.
Speaking to BBC political editor Laura Kuenssberg ahead of the start of the court case, Mr Johnson said he had the “greatest respect for the judiciary”, and its independence “is one of the glories of the UK”.
“And I think the best thing I can say, having said that, is to wait and see what they say,” he said.
On Monday, the prime minister held talks in Luxembourg with EU counterparts and negotiators.
Afterwards, he said the EU had had “a bellyful” of the Brexit process and wanted to get a deal in order to move on to the next phase of talks on future relations.
Supreme Court: Parliament suspended ‘to stop MPs frustrating PM’, judges told – BBC News