A Hong Kong executive of a publishing company known for producing books critical of the Chinese government has gone missing, according to his wife. Lee Bo is an editor working at the publisher and bookstore business Sage Communications. His disappearance comes after four of his colleagues vanished during the autumn.

There are concerns that all the five men have been secretly detained by the Chinese authorities. A source said Lee, 65, was last seen on 30 December at the publisher's Hong Kong warehouse, which he manages. Lee's wife, Sophie Choi, confirmed he was in Hong Kong on that date but went missing that evening.

"I started looking for him when he didn't come home for dinner that night at around 7pm," she said. "He called me that night at around 10pm to tell me that everything was all right." However, she said the number from which he was calling was did not belong to him and was registered in the mainland Chinese city of Shenzhen.

She reported Lee's disappearance to Hong Kong police on 31 December 2015. Hong Kong police said the incident had been listed as a "missing persons" case, according to reporting by Agence France-Presse.

When asked last autumn why his four colleagues had disappeared Lee said: "I think probably because of publishing matters… political books banned on the mainland."

Publishers threatened

The other missing men include Gui Minhai, a Swedish national and co-owner of Sage Communications. Local media said Minhai failed to return from a vacation in Thailand in October. The company's general manager Lui Bo, bookstore manager Lam Wing-kei and employee Cheung Jiping were also reported missing after vanishing in southern China during October.

Lee's disappearance is the latest incident to increase unease in Hong Kong over the rowing back of political freedoms in the former British colony. Civil liberties NGO Human Rights Watch says there is a "concerted effort" by the mainland to stop Chinese political books travelling from Hong Kong to China.

Meanwhile a publishing executive, who requested anonymity, told the Guardian newspaper: "The people behind Sage Communications have long been hated by the princelings [the Chinese political elite], absolutely hated."

The unnamed executive added that Hong Kong publishers feel threatened. "We get menacing calls from people claiming to be mainland authorities, saying we have to stop publication."