Video report by ITV News Political Correspondent Paul Brand
Prime Minister Boris Johnson's motive for an "exceptionally long" prorogation was to "silence" Parliament, Supreme Court justices have heard.
Judges sitting at the UK's highest court have been asked to rule whether the prime minister's decision to suspend Parliament was unlawful.
Laywers for Gina Miller, who is bringing a challenge over Mr Johnson's advice to the Queen to suspend Parliament for five weeks, told the judges the prime minister's decision was an "unlawful abuse of power."
They also said he has failed to provide a witness statement or any other evidence to explain his decision to the court.
Mr Johnson says the five-week suspension is to allow the Government to set out a new legislative agenda in a Queen’s Speech when MPs return to Parliament on October 14.
But those challenging his decision argue that it is designed to prevent parliamentary scrutiny of the UK's impending exit from the EU on October 31.
During a historic hearing in London on Tuesday, Mrs Miller's barrister Lord Pannick QC said: "The exceptional length of the prorogation in this case is strong evidence that the Prime Minister's motive was to silence Parliament for that period because he sees Parliament as an obstacle to the furtherance of his political aims."
The court is hearing appeals from two separate challenges brought in England's High Court and Scotland's Court of Session on the legality of prorogation - cases which resulted in two different outcomes.
The High Court in London dismissed Mrs Miller's case, finding that the length of the prorogation was "purely political" and not a matter for the courts.
In Edinburgh, the Court of Session concluded Mr Johnson's decision was unlawful because it was "motivated by the improper purpose" of frustrating Parliament.
President of the Supreme Court, Lady Hale, said in her opening statement the court is "not concerned with the wider political issues," but with answering the difficult questions of law.
Justices will consider whether it's right for the courts to rule on disputes between Parliament and the Government and if so, they will rule whether Mr Johnson's advice to the Queen was unlawful.
Lord Pannick argued the High Court "erred in law" in the case of Mrs Miller - who previously brought a successful legal challenge over the triggering of the Article 50 process to start the Brexit countdown - and the Court of Session reached the "correct" conclusion.
The barrister said the appeal raises "fundamental questions of constitutional law" and that no court had been asked to consider these issues because no Prime Minister "has abused his power in the manner in which we allege in at least the last 50 years".
But Lord Keen, the Advocate General for Scotland, arguing against the Scottish decision, said: "Insofar as the Inner House has sought to declare the prorogation unlawful, I will take no issue with their order.
"But, insofar as they purport to declare that the prorogation is null and of no effect, I submit that they have simply gone where the court could not go."
He told the court that Mr Johnson will comply with the Supreme Court's declaration in the event it concludes the five-week suspension was unlawful.
However, he refused to rule out the possibility the prime minister would advise the Queen to prorogue Parliament for a second time.
Mrs Miller's High Court challenge was supported by former prime minister Sir John Major and shadow attorney general Baroness Chakrabarti and a cross-party group of around 75 MPs and peers, led by SNP MP Joanna Cherry QC, was responsible for the Scottish challenge.
A crowd of about 40 protesters, holding signs saying "Defend democracy", "Reopen Parliament" and "They misled the Queen", gathered outside the court ahead of the hearing.
Among them was a man dressed as Robocop and another calling himself the "Incredible Sulk" - wearing a blond wig with an Incredible Hulk costume - who was inspired by Mr Johnson comparing himself to the comic book character in a recent interview.
The Supreme Court will hear submissions from the parties - including the prime minister's case that his advice on the suspension was lawful - and interveners from Tuesday to Thursday, but it is not clear when they will give a ruling.