The Supreme Court will decide on Tuesday on whether British Prime Minister Boris Johnson acted unlawfully in shutting down parliament in the run-up to the country's scheduled EU exit next month.

Britain's highest court will rule on the legality of Johnson's advice to Queen Elizabeth II to prorogue parliament for five weeks until October 14 -- just a fortnight before Brexit day on October 31.

After three days of hearings last week, a spokeswoman said the decision would be made at 10:30 am (0930 GMT), with seven of the 11 judges who were involved in the case set to attend.

Johnson, who took office in July, insists the decision to suspend parliament earlier this month was a routine move allowing his new government to launch a fresh legislative programme.

But critics accused him of trying to silence MPs, most of whom oppose his threat to leave the European Union on October 31 without a divorce deal with Brussels.

A ruling in Johnson's favour would uphold the long-held principle that prorogation is solely a decision for the prime minister, not the courts.

But many fear that, in a country without a formal written constitution, it would establish a precedent that premiers could suspend parliament for long periods.

If Johnson loses, it is likely to prompt demands for parliament to be immediately recalled.

Supreme Court President Brenda Hale last week stressed that the verdict would not determine when and on what terms Britain leaves the European Union.

But the ruling is the culmination of three years of battles over the shape of Brexit between Johnson and his predecessor, Theresa May, and the House of Commons.

In the week between returning from their summer break and parliament's suspension, MPs passed a law aimed at stopping Johnson delivering a "no deal" Brexit next month.

The Supreme Court judges are ruling on appeals against two conflicting lower court decisions on Johnson's move.

Scotland's highest civil court found the suspension was unlawful, but the High Court in England said it was not a matter for judges to intervene in.