China 'removed' Hong Kong bookseller, says Britain

China has committed a "serious breach" of a handover treaty, the British government has said after finding that a missing bookseller was likely to have been "involuntarily removed", media reports said on Friday.

In a six-monthly report to parliament, Foreign Secretary Philip Hammond said that Lee Bo, a British passport holder who disappeared from Hong Kong in late December 2015, was likely to have been taken to China against his will, The Guardian reported.

It is the first time Britain has noted a serious breach of the 1984 Sino-British Joint Declaration under which Britain is obligated to ensure China protects free speech and other legal commitments in Hong Kong.

Hammond said that while the full facts of the case are still not clear, "our current information indicates that Lee was involuntarily removed to the mainland without any due process," BBC reported.

"This constitutes a serious breach of the Sino-British Joint Declaration on Hong Kong and undermines the principle of 'One Country, Two Systems' which assures Hong Kong residents of the protection of the Hong Kong legal system," he added.

Lee is one of five booksellers or publishers missing from Hong Kong. All five are associated with publishing house Mighty Current, which specialises in books critical of senior Chinese leaders, and Causeway Bay Books which sells the titles.

Their supporters believe China has detained them because of this.

However, China has confirmed it is investigating three of the men for "illegal activities".

A fourth suspect, Swedish citizen Gui Minhai, has appeared on Chinese state TV saying he voluntarily handed himself over to the authorities over a drunk-driving fatality.