PRIME Minister Boris Johnson is set to find out the ruling of the UK's highest court over the legality of his advice to the Queen to suspend Parliament for five weeks.

A panel of 11 Supreme Court justices will announce their findings at 10:30am tomorrow following a historic hearing in London last week.

The judges have been asked to rule on whether the Prime Minister's advice to the Queen to prorogue Parliament until October 14 was unlawful.

Defeat for Johnson could see MPs returning to the Commons this week.

However, during proceedings, the UK Government argued that even if the Prime Minister loses the case Parliament could remain suspended.

Documents submitted by Sir James Eadie QC and Advocate General for Scotland Lord Keen put forward two such scenarios.

The first envisages a situation where the judges find the prorogation was unlawful, but their reasoning leaves open the possibility that Parliament could be prorogued for five weeks in a lawful manner.

A second possible outcome is that the court could find the suspension was unlawful and that the recall of Parliament before October 14 is the “only option lawfully open to the Prime Minister”.

They asserted that Parliament could even be prorogued for a second time, depending on the court's reasoning.

Lord Keen said the issue of prorogation was “a matter between the executive and Parliament”, adding that if Parliament “takes exception” to actions of the executive “they have the tools available”, namely a motion of no confidence.

Johnson is currently in New York where he is to meet US President Donald Trump tomorrow for talks at the United Nations General Assembly, where he will also meet key EU leaders.

The Supreme Court judges, led by the court's president Lady Hale, heard appeals over three days from September 17 arising out of legal challenges in England and Scotland – which produced different outcomes.

At the High Court in London, the Lord Chief Justice Lord Burnett and two other judges rejected a challenge against the Prime Minister's prorogation move by campaigner and businesswoman Gina Miller.

But in Scotland, a cross-party group of MPs and peers won a ruling from the Inner House of the Court of Session that Johnson's prorogation decision was unlawful because it was "motivated by the improper purpose of stymieing Parliament".

Miller's barrister Lord Pannick QC told the Supreme Court that Johnson's motive for an "exceptionally long" prorogation was to "silence" Parliament, and that his decision was an "unlawful abuse of power".

Sir James Eadie QC argued on the Prime Minister's behalf that the suggestion the prorogation was intended to "stymie" Parliament ahead of Brexit was "untenable".

The Prime Minister advised the Queen on August 28 to prorogue Parliament for five weeks and it was suspended on September 9.

Johnson claimed the five-week suspension was 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 who brought the legal challenges argued the prorogation was designed to prevent parliamentary scrutiny of the UK's impending exit from the EU on October 31.

At the close of the unprecedented proceedings, Lady Hale said: "I must repeat that this case is not about when and on what terms the United Kingdom leaves the European Union.

"The result of this case will not determine that.

"We are solely concerned with the lawfulness of the Prime Minister's decision to advise Her Majesty to prorogue Parliament on the dates in question."