British Prime Minister Boris Johnson has vowed to press on with his plans for Brexit despite a devastating ruling by Britain's highest court that his suspension of parliament was unlawful.

Mr Johnson said he would abide by the finding of the Supreme Court that the five-week prorogation was "void and of no effect", even though he disagreed with its conclusion.

Following the legal bombshell, Commons Speaker John Bercow announced that MPs would return to Westminster tomorrow morning.

The ruling prompted immediate demands from opposition for Mr Johnson to quit amid claims his position had become untenable.

Downing Street insisted there was no question of Mr Johnson, who is in New York for the UN General Assembly when the result was announced, stepping aside.

While Mr Johnson said the return of MPs would go ahead, he made clear his unhappiness with the court's "unusual judgment".

"I strongly disagree with this decision of the Supreme Court," he told reporters.

"I have the utmost respect for our judiciary, I don't think this was the right decision, I think that the prorogation has been used for centuries without this kind of challenge.

"I think the most important thing is we get on and deliver Brexit on October 31, and clearly the claimants in this case are determined to frustrate that and to stop that.

"I think it would be very unfortunate if parliament made that objective, which the people want, more difficult but we will get on."

Announcing the result, the court's president Lady Hale said the government's advice to Queen Elizabeth to prorogue parliament was unlawful because "it had the effect of frustrating or preventing the ability of parliament to carry out its constitutional functions without reasonable justification".

She said the prolonged suspension of parliamentary democracy took place in the "quite exceptional circumstances" of the UK's impending exit from the EU on 31 October.

She added: "Parliament, and in particular the House of Commons as the elected representatives of the people, has a right to a voice in how that change comes about.

"The effect upon the fundamentals of our democracy was extreme."

Mr Johnson said Brexit is "not made much easier" by the judgment.

"It is perfectly usual to have a Queen's speech, that is what we want to do, but more importantly let's be in no doubt there are a lot of people who want to frustrate Brexit," he said.

"There are a lot of people who want to stop this country coming out of the EU.

"We have a parliament that is unable to be prorogued, doesn't want to have an election, and I think it is time we took things forward."