Boris Johnson’s motive for proroguing the Commons for five weeks was “to silence parliament for that period”, the supreme court has been told at the opening of an emergency appeal.

The prime minister’s extended suspension of debate was carried out for an “improper purpose” in order to “avoid the risk of parliament undermining the policies of his executive”, said Lord Pannick QC, who was representing the businesswoman and legal campaigner Gina Miller.

Before the arguments formally began in central London, Lady Hale, the president of the supreme court, said it was facing “serious and difficult questions”. That was evidenced, she said, “by the fact that three senior judges in Scotland have reached a different conclusion to three senior judges in England and Wales”.

She continued: “The supreme court exists to decide such difficult questions of law and we shall do so in accordance with our judicial oath.” This declared the judges must reach their findings “without fear or favour, affection or ill will”.

Hale stressed it was not the responsibility of the court to decide “when or how the UK leaves the EU”. Judgment is not expected until the end of the week at the earliest.

Q&A What is the UK supreme court and how does it work? Show Hide The supreme court of the United Kingdom was formally established on 1 October 2009. It is the ultimate court for hearing cases of the greatest public or constitutional importance, and is the final court of appeal in the UK, except for criminal cases heard in Scotland. The supreme court took over the judicial functions of the House of Lords, separating the judiciary from the legislature. It brought an end to the system of appealing to the "lords of appeal in ordinary" - commonly known as the law lords. The court comprises the president and deputy president and 10 other judges of the supreme court. Judges are appointed by the Queen on the advice of the prime minister, to whom a name is recommended by a special selection commission. The prime minister is obliged to recommend this name. The judgments in all decided cases from the court are published on the court's website.

In his written submission to the supreme court, Pannick wrote: “The prime minister’s advice to Her Majesty to prorogue parliament for a period of five weeks is an unlawful abuse of power.

“The prime minister’s reasons for advising on a five-week prorogation were improper in that they were infected by factors inconsistent with the concept of parliamentary sovereignty, in particular his belief that parliament does nothing of value at this time of year and his concern that parliament might take steps which would undermine the government’s negotiating position with the EU.

“If the judgment of the divisional court were to be upheld, the courts would have no power to review a prime minister’s advice to prorogue parliament for a period of six months, one year or longer.” At least 19 sitting days were being lost, Pannick said.

“It is a remarkable feature of these proceedings that the prime minister has not made a witness statement explaining why he decided to advise Her Majesty to prorogue parliament for a period as long as five weeks and there is no evidence from the cabinet secretary or any other official explaining that.”

Facebook Twitter Pinterest Lord Pannick represents the businesswoman and legal campaigner Gina Miller, pictured arriving at the supreme court, in central London, on 17 September. Photograph: Tolga Akmen/AFP/Getty Images

Pannick added: “Parliament will be silenced for a substantial part of the period leading up to the deadline of 31 October, when issues of grave national importance are being addressed (or not addressed) by the government … The evidence shows that the prime minister at best improperly regards parliament as an irrelevance.”

In written submissions on behalf of the prime minister, Sir James Eadie QC and Lord Keen QC, argued the claims were “non-justiciable” because there were no “judicial or manageable standards by reference to which the courts could assess the lawfulness of [such] ministerial decisions”. They added: “It is well settled that the courts do not enforce constitutional conventions, which rest on a careful constitutional and political balance.

“It is not for the courts to devise further additional controls on parliament sittings beyond those already set out in legislation. That would be a negation, rather than affirmation, of parliamentary sovereignty.”

Pannick argued the justices were entitled to draw adverse inferences from the prime minister’s failure to provide a witness statement to the court.

Earlier, the justices and counsel spent several minutes trying to negotiate their way through mounds of paper files and electronic submissions to ensure they were all reading the same pages and case authorities. “You will have to decide whether we need an enlarged bundle,” one of the justices, Lord Wilson, said to Pannick.

Lady Black asked whether any questions tabled by MPs had not been answered. Lady Hale said it would be of “great interest to us to know what bills are lost” by the early prorogation of parliament.

Lord Pannick pointed out that the same principle of parliamentary supremacy had been accepted by Lord Reed in his landmark ruling on employment tribunal fees, in a case brought by the trade union Unison, in which he concluded that the lord chancellor had raised fees to an unlawfully high level so that access to justice was being prevented.

“My Lord [Reed] was not deterred by the difficulty of when a fee would become lawful,” Pannick observed. “How much a fee [should be] did not deter [Reed] from saying the approach of the lord chancellor was unlawful because it was denying access to the courts and that it was a breach of the rule of law.

“The basic principle is that parliament is supreme. The executive is answerable to parliament. It must also apply to cases. This is a unique case. This has never occurred before. It must apply to cases where the executive takes decision with the purpose or affect of removing parliament’s ability to legislate.

“It would be a much greater breach of parliamentary sovereignty ... than to defy a particular law. The junior body, the executive, can’t use its powers to prevent the superior body, parliament, from performing its functions.”

Lord Pannick said there had no been no previous need to establish the relevant principles of our constitutional law in the context of the power of prorogation, because no prime minister in the past 50 years had abused his powers in the manner alleged here.

The hearing, which is due to last three days, continues.