Image copyright AFP Image caption Lee Bo, also known as Paul Lee, disappeared in December 2015

The UK has said a British bookseller who has disappeared in Hong Kong was likely "involuntarily removed" to China, calling it a "serious breach" of the handover treaty.

The statement from Foreign Secretary Philip Hammond is the UK's strongest comment so far on the case of Lee Bo.

It is also the first time the UK has noted a serious breach of the 1984 Sino-British Joint Declaration.

Mr Lee is one of five booksellers or publishers missing from Hong Kong.

China has yet to issue an official response to the Foreign Office's report.

Read more: Hong Kong's missing booksellers and 'banned' Xi Jinping book

All five missing men are associated with publishing house Mighty Current, a publishing house specialising 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. China has confirmed it is investigating three of the men for "illegal activities".

A Hong Kong government spokesman said: "Any suggestion that Mr Lee was involuntarily removed to the mainland remains speculative".

The spokesman said that Hong Kong authorities had sought help from mainland China and would "continue to follow through with the cases".

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

First use of 'serious breach'

Lee Bo, also known as Paul Lee, who holds a British passport, disappeared from Hong Kong in December.

Image copyright AFP Image caption Mr Hammond also raised concern about "unprecedented pressure " on Hong Kong's rights and freedoms

Letters sent to his family, apparently by him, have said he went to China voluntarily to attend to certain matters, but did not elaborate.

The UK had earlier expressed concern, but in his twice-yearly report on Hong Kong affairs, Mr Hammond said that while the full facts of the case were still not clear, "our current information indicates that Mr Lee was involuntarily removed to the mainland without any due process".

"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.

Under Hong Kong law, Chinese police do not have jurisdiction in the territory.

It is the first time the UK has accused China of a "serious breach" of the 1984 Sino-British Joint Declaration, the document in which China promised to safeguard Hong Kong's freedoms after the territory was handed back by Britain.

A Foreign Office spokeswoman said there was no legal provision to enforce a breach, but that the UK would continue to raise Mr Lee's case with China.

Mr Hammond has previously said any charges against Mr Lee should be dealt with in Hong Kong, not mainland China.

The report, which also assessed Hong Kong's media, academic and legal progress, also said some of the city's guaranteed rights and freedoms "have come under unprecedented pressure" and urged authorities in Hong Kong and Beijing to take steps in maintaining confidence in the system.

Mighty Current publishing house disappearances

1. Lui Bo, General Manager, goes missing in Shenzhen, 15 October

2. Cheung Jiping, business manager, 32, goes missing in Dongguan, 15 October

3. Gui Minhai, co-owner, 51, goes missing in Thailand, 17 October

4. Lam Wingkei, manager, 60, last seen in Hong Kong, 23 October

5. Lee Bo, shareholder, 65, goes missing in Hong Kong, 30 December