The nature of a 2020 legislative primary election changed from selection of the best candidates for Hong Kong’s opposition camp after its driving force pushed to incorporate the unofficial event into an “insane” blueprint for “mutual destruction”, a one-time ally turned prosecution witness has told a subversion trial.

Au Nok-hin, an ex-politician, on Wednesday said it was “regrettable” that former law professor Benny Tai Yiu-ting had written a series of articles that favoured blocking government budgets and bills in the Legislative Council as a way to topple the city’s leader.

“I once thought that [whether to veto the budgets] was a choice to be made by individual candidates, but, as he gradually published articles about ‘mutual destruction’ in June , I started to feel that his views had led to changes in the nature of the primary election,” Au told West Kowloon Court.

The former Democratic Party lawmaker said he had insisted that contenders in the unofficial poll in July that year should not be forced to veto budgets regardless of their merits because of the “very high” risk of a breach of the Beijing-imposed national security law that had just come into force.

Benny Tai, alleged in court to have been the driving force behind an ‘insane’ plan to topple the chief executive through an unofficial opposition primary in 2020.

Au was speaking as he was cross-examined by defence lawyers in a 90-day trial involving 16 of the 47 opposition figures who took part in the primary.

The 16 have denied allegations that they were part of a conspiracy to subvert state power.

The 31 others, including Tai, Au and two other coordinators of the poll, have pleaded guilty or indicated they would admit liability.

The 47 were accused of organisation or participation in the primary to ensure they won a majority in the Legislative Council so they could paralyse the government and force Carrie Lam Cheng Yuet-ngor, then the chief executive, to step down.

Prosecutors earlier told the court the ­conspiracy was part of a “destructive manifesto in 10 steps to ‘mutual destruction’” inspired by Tai, who aimed to undermine the government by manipulation of the electoral system.

Tai had said he believed the primary would enable the opposition to twice veto the budgets, despite a crackdown by authorities, and force the chief executive to resign.

He claimed that Beijing would be forced to declare a state of emergency and persecute opposition leaders, which would trigger a backlash by the public and “bloody” suppression in Hong Kong that would lead to international sanctions against the Chinese Communist government.

Au earlier testified that the manifesto, first proposed in a newspaper article on April 28, 2020, was “too insane”.

He highlighted on Wednesday his unease over the road map with a reference to what Tai said would happen during the last step he envisioned.

Tai said in the April 28 article, published in the now-closed Apple Daily newspaper, that by the time the manifesto came to fruition, Hong Kong would have already “jumped off the cliff together with the Chinese Communist Party”.

“We are living in a Hong Kong governed under the ‘one country, two systems’ principle,” Au said. “For you to publish an article that inflames the country’s feelings, I believe that is too much.”

The court heard that Au withdrew from the primary in July 2020, three days after the poll.

Au appeared on a radio programme the next day and said he drew the line at the manifesto and insisted the primary was not designed to fulfil Tai’s political ambitions.

The trial continues.