A former Apple Daily publisher decided to become a prosecution witness "to tell the truth" because of his own conscience, the national security trial of the newspaper's founder Jimmy Lai has heard.

Cheung Kim-hung stressed that he was not asked by the police to become a prosecution witness.

On Thursday, the defence team said during cross-examination that the ex-publisher was visited by a police sergeant while in custody on November 11, 2021, after his bail application was rejected by the High Court.

The trial heard Cheung spent about 20 minutes reading a 10-page written decision denying him bail, handed to him at Lai Chi Kok Reception Centre by an officer he'd met for the first time.

Representing Lai, senior counsel Robert Pang suggested the meeting was six hours long, but Cheung said it lasted for about four hours, and that he mainly spoke with the officer about life in remand.

The court was told the same sergeant, along with another officer, visited Cheung again the next day, during which the former publisher said he had something to disclose to the police and wanted to be a prosecution witness.

Cheung subsequently switched his legal representatives.

“It was me, after reading the bail ruling, who wanted to tell the truth," he said.

The ex-publisher, among six former staff who admitted to conspiracy to commit collusion, also told the court that Lai’s newspaper columns and his series “Live Chat with Jimmy Lai” remained on the Apple Daily's website after the enactment of the national security law in 2020 because "they were by the boss".

Those materials were removed from the website after Lai was charged with national security offences, Cheung added.

The Apple Daily founder has denied three conspiracy charges relating to collusion with foreign forces and sedition.

The trial continues on Friday.