BEIJING • A Chinese columnist has gone missing after warning former colleagues of the danger of re-publishing an open letter calling for President Xi Jinping to resign, the journalist's lawyer told Reuters.

Mr Jia Jia, a 35-year-old freelance writer based in Beijing, who writes a regular column for Tencent Online, was reported missing on Tuesday night.

He was scheduled to board a flight from Beijing to Hong Kong, where he was due to give a lecture at the City University of Hong Kong on Thursday, reported Apple Daily, a newspaper based in Hong Kong and Taiwan.

The newspaper also quoted Mr Jia's wife, who said she last spoke to her husband at 8pm on Tuesday and he had told her that he was about to board the plane to Hong Kong.

Mr Jia was going to stay with a friend in Hong Kong, but he did not show up at the friend's home. He also missed a lunch appointment the next day, said his wife in the Apple Daily report.

Before his scheduled departure, Mr Jia had told friends that he believed something could happen to him after he warned former colleagues about re-publishing the letter, his lawyer, Mr Yan Xin, told Reuters.

Mr Yan said Mr Jia had told him that he had cautioned his former colleague, Mr Ouyang Hongliang, an editor at the Watching news agency, after Watching had re-posted the letter.

Mr Jia, a frequent commentator on political and social affairs, had also told him that he had no connection to the letter, which was signed by a "loyal Communist party member".

The letter was seen by Reuters on a cached page on Watching's website. It could not be found on Watching when Reuters checked on Thursday. Reuters was also unable to reach Mr Ouyang for comment.

The letter also circulated on Chinese social media, including the WeChat messaging app, before the authorities apparently took it down.

It was unclear whether or not Mr Jia has been taken into custody. Beijing law enforcement authorities could not be reached for comment.

His apparent disappearance comes amid mystery surrounding five Hong Kong booksellers who dealt in gossipy books about Chinese leaders, and who also went missing only to resurface in Chinese custody.

President Xi has launched an unprecedented effort to clamp down on the Internet and censor opinions that do not fall in line with those of Communist Party leaders, including imposing tougher penalties for "spreading rumours" via social media.