[LONDON] Britain's Supreme Court on Tuesday ruled "unlawful" a decision by Prime Minister Boris Johnson to suspend parliament in the run-up to Brexit, saying it was "void and of no effect".

"The court is bound to conclude... that the decision to advise Her Majesty to prorogue parliament was unlawful," Supreme Court head Brenda Hale said in the ruling.

House of Commons speaker John Bercow on Tuesday said parliament must "convene without delay" after the Supreme Court ruled that Prime Minister Boris Johnson's decision to suspend it earlier this month was unlawful.

Mr Bercow, who has been highly critical of Johnson's decision, said he would be consulting party leaders "as a matter of urgency".

Opposition Labour leader Jeremy Corbyn has called on Mr Johnson to resign but according to a Downing Street source, Mr Johnson will not do so and will fly back to London after his speech to the United Nations on Tuesday.

He will hold a call with Cabinet ministers later today, the source said. He had been due to leave New York on Wednesday.

Sterling extended gains on Tuesday while eurozone stocks tumbled after the court ruling.

The British currency gained as much as nearly 0.5 per cent on the day to $1.2491 after the ruling while stocks tumbled on the back of a stronger pound. Against the euro, the pound rose 0.3 per cent on the day to 88.06 pence.

British gilt yields rose following the decision, dragging safe-haven German bond yields higher.

British government bond futures fell to a session low of 133.12, down 60 ticks on the day, while 10-year gilt yields rose 3 basis points on the day to 0.585 per cent.

"This is a win for the pro-remain side and suggests it will be harder for the government to push a no-deal brexit before the end of October," said Chris Graham, senior economist at Standard Chartered Bank in London.

AFP, REUTERS