AN historic legal judgment which could cut short Boris Johnson’s premiership will be delivered this morning at the UK’s highest court.

Eleven Supreme Court justices will hand down a definitive ruling on whether the Prime Minister exceeded his powers by suspending Westminster for five weeks ahead of the current deadline for Brexit.

Their judgement is expected to break new constitutional ground and trigger a potentially ferocious debate over the relationship between the Government, parliament and the judiciary.

Mr Johnson will be in New York for the UN Climate Action Summit when the decision is delivered at 1030am.

READ MORE:  Jeremy Corbyn suffers Brexit blow as Unison breaks ranks and backs pro-Remain campaign

If the judges find the Prime Minister acted unlawfully when he advised the Queen to have the prorogation, it could lead to parliament being recalled early.

It would also lead to questions about whether Mr Johnson lied to the Queen when he formally requested the suspension on August 28.

The Prime Minister has claimed that proroguing parliament from September 9 to October 14, the longest suspension outside an election since 1931, was simply to tee up a new legislative session.

However two legal challenges claimed its true intent was to stop MPs debating and scrutinising his plan for a “do or die” Brexit on October 31.

Activist Gina Miller, whose action was joined by former Tory PM Sir John Major, lost an initial challenge at the High Court in London last month, after judges ruled the issue was one of politics and therefore “non-justiciable”, meaning it was not a matter for the courts.

However, she appealed to the Supreme Court, which heard her case last week.

At the same time, the Supreme Court also heard an appeal by the UK Government against a decision of the Inner House of the Court of Session in Edinburgh on the same point.

Three Scottish judges ruled earlier this month that Mr Johnson’s advice was indeed a matter for the courts, not simply one of politics, and that moreover, it was unlawful because improperly motivated.

READ MORE: Ian McConnell: SNP’s Mackay has every right to be aggrieved as Tory Brexit drags down nation 

They agreed with a challenge brought by 75 MPs and peers led by SNP MP Joanna Cherry QC that the intent was to “stymie” parliamentary scrutiny of Brexit.

Although it did not say so explicitly, the Scottish ruling implied Mr Johnson misled the Queen over the prorogation, as well as parliament and the public.

Former Tory Attorney General Dominic Grieve has said that deceiving he monarch would be a resignation issue.

Although the Supreme Court justices gave little away during the three days of hearings, many legal observers believe they will rule against the Government.

They did not appear to accept the Government’s main line of defence, that prorogation was “forbidden territory” for the courts, and the challenges should simply be thrown out.

If they had been inclined to accept that, it would probably have happened already.

Many of the justices also asked exactly what the Government would do if it lost.

This significant discussion of the potential legal “remedy” saw the Government’s lawyers admit the Prime Minister might simply decide to prorogue parliament again - but this time on a lawful footing.

In a telling phrase, Lord Keen QC, the advocate general for Scotland, who was acting for the UK Government, last week referred to the “original” prorogation.

In her closing remarks, President Lady Hale stressed the judges were “solely concerned” with the lawfulness of the PM’s advice to the Queen on the dates in question, rather than Brexit, saying it was “not a simple question”.

On Sunday, Foreign Secretary Dominic Raab said the Government would abide by the Supreme Court judgment, but refused to rule out a second prorogation.

He said: “Let’s wait and see what the first judgment decides and then we will understand the lie of the land.”

Edinburgh South Labour MP Ian Murray, one of the petitioners in the Scottish challenge, said: “This case has been brought because we believe we must uphold British democracy in the face of this attack from Boris Johnson.

“It’s disappointing that we have had to go all the way to the courts and asked judges to make a decision.

“It should never have come to this. The Prime Minister’s decision to shut down parliament at this crucial moment for our country was disgraceful.

“But whatever happens, we are now closer than ever to a parliamentary majority for a final say on Brexit, with the opportunity to remain in the EU.”