The Court of Final Appeal (CFA) on Monday refused to hear an appeal by the Department of Justice (DoJ) against a lower court's decision to allow a British barrister to represent former media tycoon Jimmy Lai at his upcoming national security trial.

It was the last chance for the government to secure a court ban on King's Counsel Tim Owen defending Lai at the trial which is scheduled to begin on December 1.

Government counsel Rimsky Yuen told the top court that allowing an overseas barrister in the case would go against the objective of the national security law to prohibit foreign forces from intervening in Hong Kong affairs.

He also said there was no guarantee that an overseas lawyer would protect state secrets.

However, in handing down its decision, a three-judge panel said the arguments presented to the CFA had not been put forward when the issue was heard in the High Court and Court of Appeal.

"The secretary for justice had fundamentally changed his case only at the stage of seeking leave to appeal to the Court of Final Appeal, had raised undefined and unsubstantiated issues said to involve national security which were not mentioned or explored in the courts below, and no appropriate basis had been made out for the grant of leave to appeal," the judges said.

Lai is accused of taking part in a conspiracy to print, publish, sell, offer for sale, distribute, display and/or reproduce seditious publications, as well as conspiring with others to collude with a foreign country or external elements to endanger national security.