HONG KONG (Reuters) - A British activist critical of Hong Kong’s rights record was barred entry to the former British colony on Wednesday, prompting a demand from London for an explanation, a week before a Communist Party leadership meeting starts in Beijing.

Benedict Rogers, a co-founder of the Conservative Party’s Human Rights Commission, has been a vocal critic of Chinese-ruled Hong Kong’s treatment of human rights activists, including that of jailed student protest leader Joshua Wong.

After arriving from Bangkok on Wednesday, Rogers said immigration officials who were “perfectly friendly and polite” took him into a room and briefly asked him non-sensitive questions.

They denied him entry about an hour later without giving a reason, Rogers told Reuters over the phone. He was then escorted on to the next flight to Bangkok by half a dozen officials.

Hong Kong, which returned to Chinese rule in 1997, is governed under a “one country, two systems” principle that promises it a high degree of autonomy and freedoms not enjoyed on the mainland.

But critics have accused the government of bending to the will of Communist Party leaders in Beijing and of a gradual watering down of the territory’s freedoms, including freedom of speech and right to protest.

The Immigration Department said in a statement it does not comment on individual cases, adding it decides whether entry will be allowed “in accordance with the Hong Kong law and prevailing immigration policies”.

The city’s leader, Carrie Lam, also said she would not comment on individual cases when asked about it at a press conference about her maiden policy address.

British Foreign Secretary Boris Johnson said London needed an explanation from Hong Kong and Beijing.

“I am very concerned that a UK national has been denied entry to Hong Kong. The British Government will be seeking an urgent explanation from the Hong Kong authorities and from the Chinese Government,” he said in a statement.

“Hong Kong’s high degree of autonomy, and its rights and freedoms, are central to its way of life and should be fully respected.”

Hong Kong has, on occasion, barred entry to dissidents, including former leaders of the 1989 Tiananmen Square protests in Beijing and a Dutch sculptor who made a Tiananmen sculpture, though other Tiananmen activists have been allowed in for short visits.

Rogers said he was “shocked” but he had been prepared because two days ago he had received, through an intermediary, a series of messages from the Chinese Embassy in London expressing displeasure with his visit.

“They actually described me, to my amazement, as a ‘grave threat to China-British relations’,” Rogers said.

Rogers said the embassy had also become aware of his private discussions with others on the possibility of visiting Wong, leader of the 2014 pro-democracy protests that brought much of Hong Kong to a standstill for weeks, in prison, even though the plan fell through.

He said a few messages relayed to him via the intermediary subsequently became more threatening, though they were not physically threatening.

“I’m also shocked because one aspect of ‘one country, two systems’ is Hong Kong people governing Hong Kong,” Rogers said.

“But this decision was not made in Hong Kong. This decision was from the Chinese government. This raises serious questions about that particular aspect of ‘one country, two systems’.”

The Chinese Embassy did not immediately respond to a request for comment.

China’s Communist Party holds a key party congress this month, a five-yearly event at which President Xi Jinping is expected to tighten his grip on power.