London: In a major setback for Prime Minister Boris Johnson, Britain's Supreme Court on Tuesday ruled in a historic verdict that his decision to suspend Parliament in the run-up to Brexit was "unlawful".

Johnson suspended, or prorogued, Parliament for five weeks earlier this month, saying it was to allow for a Queen's Speech to outline policies of his new government.

However, Opposition MPs and many members of his own Conservative Party had accused him of trying to escape parliamentary scrutiny during a crunch phase ahead of the October 31 Brexit deadline.

Indian-origin anti-Brexit campaigner Gina Miller had challenged his decision in the UK High Court, which had referred it to the highest court of the country.

"The decision to advise Her Majesty to prorogue parliament was unlawful," Supreme Court President Lady Brenda Hale said, as she handed down the verdict on Tuesday.

"The effect on the fundamentals of our democracy was extreme," she said.

She said the unanimous decision of the 11 justices was that Parliament had not been prorogued the decision was null and of no effect and it was for the Speakers of the Commons and Lords to decide what to do next.

It marks a major setback for Johnson, who is currently in the US for the United Nations General Assembly. He had insisted that courts should not intervene in such political matters.