In a unanimous ruling of 11 judges, the Supreme Court said that the unlawful nature of the advice given by the prime minister rendered the subsequent prorogation of parliament "unlawful, null and of no effect".
On behalf of the 11 judges, Supreme Court president Lady Hale said: "It follows that parliament has not been prorogued and that this court should make declarations to that effect. We have been told by counsel for the prime minister that he will 'take all necessary steps to comply with the terms of any declaration made by the court' and we expect him to do so. However, it appears to us that, as parliament is not prorogued, it is for parliament to decide what to do next."
In reaching its ruling on the lawfulness of the government's advice the Supreme Court first determined that the matter of whether the prime minister’s advice to the Queen was lawful was justiciable in a court of law. The government had argued that the matter was a political issue that the courts could not intervene on.
The government separately argued that even if the matter was justiciable that its advice was lawful. It said prorogation of parliament was necessary to enable it to prepare a fresh legislative agenda, but the Supreme Court found that this reason did not justify the move to prorogue in this case.
"It is impossible for us to conclude, on the evidence which has been put before us, that there was any reason – let alone a good reason – to advise Her Majesty to prorogue parliament for five weeks, from 9th or 12th September until 14th October," Lady Hale said. "We cannot speculate, in the absence of further evidence, upon what such reasons might have been. It follows that the decision was unlawful."
The Supreme Court's ruling follows separate legal action taken before the High Court of England and Wales, the Court of Session in Edinburgh and the High Court in Northern Ireland to challenge prorogation.
While the High Court in London and High Court in Belfast separately ruled that the issue was not justiciable, the Inner House of the Court of Session ruled that it was, and that the prime minister had acted unlawfully.
The Supreme Court considered appeals from both the High Court in London and the Court of Session in Edinburgh together before reaching its definitive judgment on the issue.
Litigation expert Jim Cormack QC of Pinsent Masons, the law firm behind Out-Law, said: "In reaching this decision the Justices had to address three key questions which were debated before them at length during the long hearing. The first question was whether this was a legal matter for the courts at all. The government had urged the Justices to treat this as forbidden territory, a matter of politics not a matter of law at all. The Justices had little difficulty in dismissing this argument by holding that they were not being called upon to enter politics but to protect the principle of parliamentary sovereignty which is a legal principle at common law under our constitution by setting, in law, the limit of the power of the prime minister."
"The second question was the legal test to be applied in setting the limit. The court gave a clear answer that the limit is that prime minister must not seek to prorogue parliament if it would have the effect or frustrating or preventing, without reasonable justification, the ability of parliament to pass legislation and place the government under scrutiny. It is fundamental to note the focus on the effect of suspension and not the motive of the prime minister. The court ruled motive out of consideration. It is the effect on the functions of parliament which matters and what justification is advanced," he said.
"The third question was whether the limit of power had been exceeded and the Justices held that clearly it had been in this case. Parliament had been sought to be suspended for five weeks in a critical period for the UK running up to the current 31 October Brexit deadline and the government had simply failed to advance any evidence to justify a prorogation of that length at this time. It was therefore inevitable that as a matter of law, the attempted suspension would be ruled unlawful and void. The court said that the order issued to suspend parliament was legally like 'a blank piece of paper'," Cormack said.
"Those who say that the Supreme Court entered the realm of politics should pay close attention to the reasoning of the court which is plainly to the contrary. As the Justices noted, what they have done is to apply common tools of legal analysis to place a limit on the discretion of the prime minister in a way which protects, rather than undermines, the separation of powers in our constitution among parliament, government and the courts," Cormack said.
"The Supreme Court has restricted the prime minister making it very difficult for him to attempt to prorogue again in the run up to the Brexit deadline. In the eyes of the courts he could very well be seen to be acting unlawfully on a second occasion by interfering with parliamentary scrutiny without a clear justification during an unprecedented and critical period for the future of the UK," Cormack said.