A new Hong Kong political party has called for a return to British rule as a means of achieving independence from China, amid fears that Beijing is tightening its grip in the city.

Activists launched the new party – the second to actively campaign for Hong Kong independence – on Wednesday after the city’s chief executive said he had written to Chinese leaders about the case of five local booksellers.

Leung Chun-ying said on Tuesday that he had contacted Beijing after bookseller Lam Wing-kee gave explosive details of him being blindfolded and detained for eight months deep inside the Chinese mainland.

Mr Lam’s revelations come amid heightened concern in Hong Kong that Beijing is eroding freedoms in the city that were guaranteed ahead of the return of the former British colony to China in 1997.