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.
But a government lawyer said the PM was “entitled” to suspend Parliament, and it was not a matter for the courts.
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.
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”, and he 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.”