Britain said Friday a Hong Kong bookseller believed detained by China was "involuntarily removed to the mainland", in its strongest comments yet on a case that has rocked the city amid fears its promised freedoms are being eroded.

The disappearance of Lee Bo, who holds a British passport and published books critical of Chinese politics, was a "serious breach" of an agreement signed with Beijing before the city was handed back to China in 1997, said foreign secretary Philip Hammond in a new report.

Four other booksellers from the Mighty Current publishing house also disappeared in October and the Chinese authorities have confirmed they are now under criminal investigation.

There are still question marks over what has happened to Lee, 65, 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 lawmakers and activists have accused Chinese authorities of snatching Lee from Hong Kong, contravening the semi-autonomous city's laws which do not allow Chinese 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 law," said Hammond in a regular six-month report on Hong Kong to the UK parliament.

"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," said Hammond.

The foreign secretary 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".

He urged the authorities in China and Hong Kong to reassure residents that law enforcement "is exclusively the responsibility of the Hong Kong SAR authorities, and that the fundamental rights and freedoms of Hong Kong residents will continue to be fully protected".

However, the Hong Kong government questioned Friday Britain's claim Lee had been "involuntarily removed".

The city's authorities insisted they attached "great importance to the cases" of the missing booksellers, adding Hong Kong police had continued their investigations even after Lee's wife retracted a report on her husband's disappearance.

"Any suggestion that 'Mr Lee was involuntarily removed to the Mainland' remains speculative," a government spokesman said.

"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.

Lawmaker Albert Ho, a prominent pro-democracy politician who has accused the mainland authorities of kidnapping Lee from Hong Kong, said Britain's comments would up 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 it would be echoed by other nations in the international community.

Britain had previously expressed concern over the case.

Earlier this month Washington also called on Beijing 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 under the joint agreement.

But there are growing fears those freedoms are being eroded, 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's conclusion.

"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 which left more than 100 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 clashes since pro-democracy rallies brought parts of the city to a standstill in 2014.