Lee Bo's case aroused particular concern because he disappeared in Hong Kong and did not pass through immigration at the border with mainland China
Beijing blasted Britain Friday for suggesting that a Hong Kong bookseller believed detained by China was "involuntarily removed to the mainland", accusing the former colonial power of interfering in Chinese domestic affairs.
Britain had earlier released a report describing the disappearance of Lee Bo, who holds a British passport and published books critical of Chinese politics, as a "serious breach" of an agreement signed with Beijing before Hong Kong was handed back to China in 1997.
They were Britain's strongest comments yet on a case that has rocked Hong Kong, adding to growing fears that freedoms are being eroded in the semi-autonomous city.
Beijing hit back, slamming London for making "groundless accusations against China".
"Hong Kong affairs are China's domestic affairs," foreign ministry spokesman Hong Lei said in a statement late Friday.
"We ask the British side to mind its words and actions and stop interfering in Hong Kong affairs."
Four other booksellers from the Hong Kong-based Mighty Current publishing house also disappeared in October and Chinese authorities have confirmed they are now under criminal investigation.
But questions remain over what has happened to Lee, 65, who went missing in December -- the only publisher to have disappeared from Hong Kong.
Letters purportedly written by Lee and sent to his wife confirmed he was now on the mainland and said he had gone to China of his own volition to help with unspecified investigations.
But Hong Kong lawmakers and activists have accused Chinese authorities of snatching Lee from the city, contravening laws that do not allow mainland police to operate within the territory.
"The full facts of the case remain unclear, but our current information indicates that Mr Lee was involuntarily removed to the mainland without any due process under Hong Kong SAR (Special Administrative Region) law," said Britain's foreign secretary Philip Hammond in a regular parliamentary report on Hong Kong.
"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."
Hammond said Britain had called for Lee's immediate return to Hong Kong and had been in communication with the Chinese government "at the highest level".
However, the Hong Kong government questioned Britain's claim that Lee had been "involuntarily removed".
"Any suggestion that 'Mr Lee was involuntarily removed to the Mainland' remains speculative," a government spokesman said.
Seeking to quell rising concern over the disappearances, city authorities insisted they attached "great importance to the cases" of the missing booksellers.
"The (Hong Kong government) will continue to follow through with the cases."
- 'Reasonable conclusion' -
Booksellers Lui Por, Cheung Chi-ping and Lam Wing-kee disappeared in southern mainland China in October.
A fourth missing member of the company, Gui Minhai, a Swedish national, was paraded weeping on Chinese state television in January, where he said he had turned himself in for a fatal driving accident 11 years ago.
Gui had failed to return to Hong Kong from a holiday in Thailand in October.
Albert Ho, a prominent pro-democracy politician who has accused mainland authorities of kidnapping Lee from Hong Kong, said Britain's comments would raise the pressure on authorities in the city and Beijing.
"There could be no other conclusion for any reasonable person to draw," said Ho, adding he believed the accusation would be echoed by other nations.
Britain had previously expressed concern over the case.
Washington also called on Beijing this month to explain the disappearances, with a State Department spokesman saying the incidents "raise serious questions about China's commitment to Hong Kong's autonomy".
Hong Kong enjoys freedoms unseen on the mainland, protected for 50 years until 2047 under the joint agreement.
But there are growing fears those freedoms are under threat, with attacks on journalists and interference in the city's education institutions exacerbating concern.
"There are specific grounds for serious concern around rights and freedoms," Hammond said in the report.
"We assess that some of these rights and freedoms have come under unprecedented pressure during the reporting period," he added, saying that was harmful to confidence in the 'one country, two systems' set-up.
Running battles between young Hong Kongers and police Monday and Tuesday, in which 100 were injured, were a violent reminder of the simmering tensions in the city.
Those clashes were sparked by government officials patrolling illegal street hawkers, with protesters gathering to protect the stalls from being cleared.
They were the worst unrest since pro-democracy rallies brought parts of the city to a standstill in 2014.