Foreign Secretary Boris Johnson has said he is "very concerned" after a British human rights activist was denied entry to Hong Kong.
Benedict Rogers has been a vocal critic of Chinese-ruled Hong Kong's human rights record.
Mr Johnson said he would seek an "urgent explanation" from the Hong Kong authorities and the Chinese government.
Mr Rogers told Reuters he was not given a reason for being denied entry and was escorted on a flight back to Bangkok.
He is co-founder of the Conservative Party's Human Rights Commission.
Mr Johnson said: "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.
"Hong Kong's high degree of autonomy, and its rights and freedoms, are central to its way of life and should be fully respected."
The city was handed back from British to Chinese rule in 1997.
Beijing agreed to govern it under "one country, two systems", granting the city its own legal system, limited democracy with multiple political parties and rights such as freedom of assembly and free speech.
But China's growing influence has been met with unease and concerns that the mainland could undermine Hong Kong's more politically liberal traditions.
Activists have been campaigning for years for Hong Kong to have more political freedom.