The High Court has rejected a bid by former media tycoon Jimmy Lai to challenge the authorities' decision to block a British lawyer from representing him in an upcoming national security trial.

The Committee for Safeguarding National Security earlier ruled that letting Timothy Owen take part in Lai's trial for alleged conspiracy to collude with foreign forces would pose a risk to national security, instructing immigration officials to reject work visa applications from the barrister.

A related development sparked by Lai's case saw Beijing rule that national security suspects need the chief executive's permission to hire foreign lawyers for their trials.

In seeking a judicial review of the work visa decision, Lai's legal team argued that the national security committee had overstepped its powers.

But in a written ruling handed down on Friday morning, High Court judge Jeremy Poon said Hong Kong courts do not have jurisdiction over the committee's work, its decisions are "not amenable to judicial review", and Lai's bid to challenge the visa move was "plainly and wholly unarguable".

The judge noted that the security committee is under the direct supervision and control of the central government.

"The HKSAR courts, as courts of a local administrative region, are not vested with any role or power over such matters of the [central government] because they clearly fall outside the courts' constitutional competence assigned to them under the constitutional order of the HKSAR," Poon wrote.

He added that it is "self-evident" that the duties and functions of the committee are "matters well beyond" the institutional capacity of the city's courts.

The judge rejected a claim by Lai's lawyer, Senior Counsel Robert Pang, that there would be no effective control over the committee's work and "any person aggrieved by its decision will have no recourse or remedy", if such decisions are not subject to judicial review by the SAR's courts.

"[Pang] gave some very extreme but unrealistic examples of how the [national security committee] might abuse its powers if there were no judicial rein by the courts. These fanciful and indeed alarmist remarks must be dismissed as well," Poon said.

The court also dismissed a separate legal challenge by the former media tycoon seeking to contend that an interpretation of the national security law by the National People's Congress Standing Committee late last year does not affect his trial, saying that it became "academic" when his application for the judicial review of the work visa decision was refused.

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. The trial is due to start later this year.