Gina Miller is seeking an order, which would force the government to recall Parliament.
The UK's Supreme Court on Tuesday began hearing arguments in a landmark case focussed on Prime Minister Boris Johnson's decision to suspend Parliament as part of an appeal by Indian-origin anti-Brexit campaigner Gina Miller.
Ms Miller had appealed in the High Court, which ruled that the issue did not fall within the purview of the courts but granted her permission to appeal resulting in the unprecedented Supreme Court sitting of 11 judges.
They are simultaneously also hearing the UK government's appeal against a contradictory ruling by Scottish High Court judges who had declared the Parliament suspension "unlawful".
"It is so much more important even than Brexit. It is about how we are governed, about preserving our ancient democratic freedoms, and trying at all costs to stop a dangerous precedent being created that threatens constitutionally, politically and economically to impoverish us all," said Ms Miller, in reference to her appeal.
"Proroguing Parliament is, if we don't look out, the Prime Minister's right to do whatever he so wishes, no matter how fierce or strong the opposition to it may be," said the 53-year-old investment fund manager, born Gina Nadira Singh in British Guiana (now Guyana) to Guyana's former Attorney-General Doodnauth Singh.
Boris Johnson maintains it was right and proper to terminate the last session of the Parliament, which rose last Tuesday, 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 by the October 31 deadline, if necessary without a deal.
But the Opposition parties and some members of his own Conservative MP questioned his motivation behind advising Queen Elizabeth II to suspend Parliament.
An embattled Boris Johnson was even forced to deny that he had lied to the Queen by masking his real intentions behind seeking the suspension, formally referred to as prorogation of Parliament.
Lawyers for Gina Miller and joint campaigners challenging the suspension told the Supreme Court on Tuesday there was "strong evidence" the British PM saw MPs "as an obstacle" and wanted to "silence" them.
Lady Brenda Hale, the President of the Supreme Court and Britain's senior-most judge, opened the hearing by highlighting that the case would only deal with the "serious and difficult questions of law" and not determine "wider political questions" relating to the Brexit process.
Lord David Pannick, a House of Lords peer and senior barrister representing Gina Miller in the Supreme Court appeal told the judges that the facts showed that Boris Johnson 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".
"The basic principle is that Parliament is supreme. The executive is answerable to Parliament," he said.
The case is scheduled until Thursday, following which a ruling is expected depending on whether the judges reserve their judgment to be handed down at a later date.
The Supreme Court's decision is expected to set a precedent in relation to the suspension of Parliament.
It marks only the second time that 11 justices are sitting in a UK Supreme Court case - the first time also involved Gina Miller in her successful challenge against then Prime Minister Theresa May to trigger Article 50 to start the process for leaving the European Union (EU) without parliamentary approval.
The judges will determine whether prorogation of Parliament is a matter for the courts, and if so, will go on to rule definitively on whether Johnson''s true motive was to undermine MPs' ability to legislate and respond to events as the country prepares to leave the EU.
Gina Miller is seeking a mandatory order, which would effectively force the government to recall Parliament.
"What we are witnessing is an historic attempt at a power grab by the executive, from the people. Johnson, as prime minister, wants to put himself above the law," said Gina Miller.
"The judges, meanwhile, have to ask themselves one simple question: is what Johnson is doing reasonable," she questioned.