The High Court has postponed the national security trial of former media tycoon Jimmy Lai for the third time, with the proceedings now expected to get underway on December 18.

The trial, to be presided over by national security judges Esther Toh, Susana D'Almada Remedios and Alex Lee, was originally slated to open in December 2022.

It was put back a couple of weeks in anticipation of a ruling from the National People's Congress Standing Committee regarding Lai's attempt to engage a British barrister.

The trial was later postponed for a second time – until September 25 this year – as the court awaited Beijing's decision.

In a pre-trial review hearing on Friday, the three judges put the trial back once more, to give both the defence and prosecution more time to prepare.

Lai, the founder of the now-defunct Apple Daily, is accused of colluding with foreign forces to endanger national security and conspiring to print seditious publications.

The trial is expected to last 80 days.

Lai has been remanded in custody since December 2020.