Lee Bo
Placards demand the whereabouts of bookseller Lee Bo who disappeared on 30 DecemberGetty Images

British citizen Lee Bo is one of five Hong Kong booksellers who sold titles critical of China's leaders who have recently gone missing. The 65-year-old, also known as Paul Lee, was associated with Mighty Current Media, which operates from a bookstore in the Causeway Shopping District and publishes titles highly critical of China's authoritarian Communist government.

In early 2012, Lee's wife, Choi Ka-ping founded the publishing house with a German man, who later transferred his shares to Giu Minghai and Lui Por, the South China Morning Post reported citing company records. Both Gui and Lui are among those missing.

Bo is believed to have opened the bookshop around the same time as the publishing house was founded.

Though the banned books trade was lucrative during the scandal of disgraced Politburo member Bo Xilai, who was sentenced to life imprisonment after being found guilty of corruption, a colleague of Lee told the newspaper that in recent years the business was struggling, and Lee had been forced to sell some of his holdings in the shop.

The British government said it was "deeply concerned" about the detentions.

"We can confirm that one of the individuals is a British citizen and we have urgently requested the Hong Kong and mainland authorities' assistance in ascertaining this individual's welfare and whereabouts," said a spokesman.

Although Lee holds dual citizenship as a resident of former British protectorate Hong Kong, this will count for little with Chinese authorities, who do not acknowledge dual nationality.

Lee's whereabouts remain a mystery, with the bookseller last seen at the Mighty Current warehouse on 30 December.

He contacted his wife claiming that he was assisting with an unspecified police investigation in Shenzen, mainland China. His colleague, however, said that Lee's travel permits remain in his home, and there is no official record of him having left Hong Kong.

His wife has called off a police investigation into his whereabouts, claiming he has been in contact by telephone. However, colleagues claim the calls were made under duress.

The disappearances have increased fears of a crackdown on the liberties agreed for Hong Kong citizens, as part of the deal brokered between Beijing and the British government for returning the region to Chinese control, with bookstores reportedly pulling volumes from shelves deemed potentially offensive to Beijing authorities.