Boris wanted to silence MPs: SC
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 would not determine "wider political questions" relating to the Brexit process.
Over the next three days, the Supreme Court will consider two separate legal challenges over whether Mr Johnson acted lawfully in advising the Queen to prorogue Parliament.
He 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.
Arguments from the English and Scottish cases are being heard on Tuesday, while the government's lawyers will respond on Wednesday.
Lord Pannick, the crossbench peer and QC representing Ms Miller, told the Supreme Court 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".
Inviting the court to take a negative view of 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 Parliamentâ€¦
"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."
He said he disagreed with the High Court's judgement that the issue was outside the scope of the courts, arguing the issue of whether prerogative powers were exercised legitimately was clearly a matter of public law.
"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.'
"The prime minister cannot have a discretion over the breadth of powers he enjoys."
He suggested the Court's decision would set a precedent and a future prime minister might feel justified in trying to suspend Parliament for six months or longer.
It is only the second time that 11 justices will sit in a Supreme Court case - 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.
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.
Mr Johnson has declined to say what he would do if the government loses its appeal, but 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".
The prime minister - who has been discussing Brexit in a phone call with Germany's Angela Merkel - has claimed the EU has 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.