The unanimous decision by the court's 11 presiding judges on Tuesday thrusts Britain's exit from the European Union further into turmoil as it undermines Johnson and gives legislators more scope to oppose his Brexit plans.

"The decision to advise Her Majesty 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," Supreme Court President Brenda Hale said, reading out the historic decision.

"Parliament has not been prorogued. This is the unanimous judgment of all 11 justices," she added. "It is for parliament, and in particular the speaker and the (House of) Lords speaker, to decide what to do next."

The speaker of parliament's House of Commons, where Johnson has lost his majority and most lawmakers oppose his promise to leave the European Union with or without a deal by October 31, said the chamber must convene without delay.

"I welcome the Supreme Court's judgment that the prorogation of Parliament was unlawful," said the speaker of the House of Commons, John Bercow.

"As the embodiment of our parliamentary democracy, the House of Commons must convene without delay. To this end, I will now consult the party leaders as a matter of urgency."

Sterling initially rallied by about a third of a cent against the US dollar after the news, before paring gains slightly, and at 1005 GMT stood 0.25 per cent up on the day at $US1.2460.

Parliament was suspended, or prorogued in the formal term, from September 10 to October 14. The prorogation was approved by Queen Elizabeth, Britain's politically neutral head of state, on the advice of the prime minister.

Some lawmakers, including those thrown out of Johnson's Conservative Party for rebelling against his Brexit plans, had said he should resign if he was found to have misled the queen.

"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," the judges said in their ruling.

British Labour Party leader Jeremy Corbyn called on Johnson to consider his position and call a new election.

"I invite Boris Johnson, in the historic words, to 'consider his position'," Corbyn told delegates at Labour's annual conference in Brighton.

To huge cheers and chants of "Johnson out!", Corbyn said the British prime minister should become the shortest-ever serving leader and that Labour was ready to form a government.

Australian Associated Press