Johnson, who will get the news while attending the United Nations General Assembly in New York, has refused to say whether he will resign if he is found to have broken the law, or will seek to shut down Parliament again.
Britain's highest court plans to announce the decision at 10:30 a.m. (0930GMT) after holding three days of hearings last week before 11 judges. The court is deciding whether Johnson acted improperly by shutting down Parliament this month for five weeks before Britain's Oct. 31 Brexit deadline, when the country is scheduled to leave the European Union. The topic has deeply divided British politicians and the public.
The government says the decision to suspend Parliament until Oct. 14 was routine and not related to Brexit. It claims that under Britain's unwritten constitution, it is a matter for politicians, not judges, to decide.
The government's opponents argued that Johnson illegally shut down Parliament just weeks before the country is due to leave the 28-nation bloc for the "improper purpose" of dodging lawmakers' scrutiny of his Brexit plans. They also accused Johnson of misleading Queen Elizabeth II, whose formal approval was needed to suspend the legislature.
Johnson insisted lawmakers would have "bags of time" to scrutinize his Brexit plans before Oct. 31, and told critics to "give me a break." "Parliament has had three years to discuss this issue — and will be able to come back and discuss Brexit after the European Council (summit) on October the 17 and 18,"Johnson told reporters traveling with him to the U.N. "So 'Donnez-moi un break' is my message to those who say that we have no parliamentary scrutiny. It's absolute nonsense."
Johnson, an outspoken Brexit advocate who is willing to leave the EU without a deal if necessary, has been at odds with Parliament, which has passed a law requiring the government to seek an extension to the Brexit deadline if no Brexit deal is reached by Oct. 19.
He has vowed not to do that, though he also says the government will obey the law. The suspension of Parliament sparked several legal challenges, to which lower courts have given contradictory rulings. England's High Court said the move was a political rather than a legal matter, but Scottish court judges ruled that Johnson acted illegally "to avoid democratic scrutiny."
Johnson has refused to say whether he would resign if he was found to have misled the queen. He has also not ruled out seeking another suspension of Parliament. "All I can tell you is that I have the highest regard for the judiciary in this country, I will look at the ruling with care," he said.
Associated Press writer Gregory Katz reported this story in London and AP writer Jill Lawless reported from New York.