HONG KONG (AFP/REUTERS) - British Foreign Secretary Philip Hammond warned over threats to Hong Kong's autonomy on Friday (April 8) as he visited the city where fears are growing that Beijing's grip is tightening, as he cited the example of a Hong Kong bookseller who disappeared from the city under mysterious circumstances late last year.

Saying Mr Lee Bo had been removed under duress, Mr Hammond said that the business community was "unnerved".

Mr Lee disappeared last December from Hong Kong and surfaced in China almost three months later.

He has since returned to Hong Kong and said he had not been kidnapped by the Chinese authorities as many suspect. He has also said he would renounce his British citizenship.

But Mr Hammond disputed Mr Lee's explanation. "On the basis of the evidence available to us, we are clear that he was removed from Hong Kong under duress," Mr Hammond told a small group of reporters in Hong Kong.

"He is a British citizen. We have a consular duty towards him and our principal concern now is to ensure that he is returned to Hong Kong free of any duress, to carry on his life here without any constraints or impositions on him."

There was no immediate response to a fax and e-mail seeking comment from China's Foreign Ministry regarding Mr Hammond's remarks.

Mr Hammond also said the incident had rattled the business community and undermined confidence in the city's rule of law that has long been one of the pillars of the international investment and financial hub.

"There are people in the business community who are unnerved by this incident and we need everybody to make very clear that this is not going to happen again ... that this can be regarded as an isolated incident, not any kind of systemic issue."

Despite repeated requests, however, Mr Hammond said British diplomats had not yet met with Mr Lee, nor had they received a formal request from Mr Lee to renounce his British citizenship.

Mr Hammond's visit was the first by a British foreign secretary for five years and comes in the wake of the high-profile case of a group of booksellers who went missing from Hong Kong and surfaced in China. One of them was a British citizen.

The city has been semi-autonomous since it was handed back to China by Britain in 1997 under an agreement that protects its freedoms for 50 years. However, there are concerns those freedoms are disappearing.

"Although the 'One Country, Two Systems' model is generally working well in Hong Kong, concerns have been raised over the recent booksellers' case," Mr Hammond said in a statement on Friday.

"Rule of law is the cornerstone of an open and fair society. Hong Kong's success is underpinned by its independent judiciary and respect for rights and freedoms," he added.

In a meeting with Hong Kong's leader Leung Chun Ying, the British Foreign and Commonwealth Office (FCO) said that Mr Hammond would restate "support for Hong Kong's high degree of autonomy, rights and freedoms", as well as commitment to the Sino-British joint declaration, which protects Hong Kong's liberties.

The five booksellers worked for a Hong Kong publishing house famous for salacious titles about high-ranking Chinese politicians. They went missing at the end of last year.

Four are under criminal investigation in the mainland - the fifth, Mr Lee, says he is "assisting" with the investigations and has come back to visit Hong Kong recently, insisting he is a free man.

Britain has voiced anger over Mr Lee - the only bookseller who disappeared from Hong Kong - saying it believed he was "involuntarily removed to the mainland" in what it called a "serious breach" of the handover agreement.

China criticised Britain for interfering in its affairs.

The other four men disappeared from Thailand and mainland China.

The FCO added that Mr Hammond would also "underline the importance of One Country, Two Systems and of restarting progress on political reform" in his meeting with Leung later Friday.

The political reform process has stalled since mass pro-democracy protests in 2014 failed to win concessions from the Hong Kong authorities and Beijing.

The rallies were calling for fully free leadership elections, after the government introduced a reform package that activists derided as fake democracy because it allowed Beijing to vet candidates.

The package was eventually voted down in the legislature and the reform debate is now on ice.

Mr Hammond's visit is the first stop on an East Asia tour, ahead of the Group of 7 foreign ministers' meeting in Japan starting on April 10.

He will also meet members of Scotland's Rugby Sevens team as the Hong Kong Sevens tournament kicks off on Friday, as well as Hong Kong and British businesses "to discuss new ways of connecting the UK, Hong Kong and China markets", the FCO said.