HONG KONG (REUTERS) - Hong Kong's leader denied on Friday (Oct 13) that China had taken over immigration controls in the city after a British activist was refused entry, but she couldn't rule out barring even the last colonial governor and frequent China critic, Chris Patten.

Chief Executive Carrie Lam defended Hong Kong's autonomy amid deepening questions over the Chinese-ruled financial hub's reputation as a free and international city, after a Conservative Party rights activist, Benedict Rogers, was barred from entering on Wednesday.

When asked on a radio talk show whether Patten, who has criticised perceived Chinese interference in Hong Kong affairs, would be the next to be barred, Lam stopped short of ruling it out.

"I can't exclude any possibilities because immigration matters will change depending on the case," she told public broadcaster RTHK.

Patten, who spoke extensively during a recent Hong Kong visit and was critical of the jailing of several democracy activists, has angered some in the pro-Beijing establishment.

Britain handed Hong Kong back to Chinese rule in 1997 amid promises that its widespread freedoms and autonomy would be respected under a "one country, two systems" formula.

But China's Foreign Ministry said it had the right to allow or refuse any persons wanting to enter Hong Kong, in response to Rogers' case, as this was a matter of state sovereignty.

The promise of autonomy is enshrined in the mini-constitution, known as the Basic Law, that creates a separate Hong Kong legal and policing regime, including immigration controls. But China has sovereign control over defence and foreign affairs - which critics fear Beijing is exploiting to erode the city's freedoms.

"I want to clarify any worries ... that this whole matter of immigration is now being taken over by the Central People's Government. That's certainly, definitely not the case," Lam said.

"The case has to be regarded and treated and fall under what constitutes a foreign affairs matter." Patten did not immediately respond to a Reuters request for comment.

China lodged a diplomatic complaint with London after British Foreign Secretary Boris Johnson demanded an urgent explanation from Hong Kong and China for Rogers' expulsion.

The case has also sparked outcry from the city's pro-democracy activists, including former number two official Anson Chan, who said Beijing was using the case to send a message.

Local English newspaper The Standard wrote in an editorial on Friday that "if the situation is allowed to evolve into such a state that whoever disliked is barred, it could be the moment we begin to lose our international appeal".

The US Consulate in Hong Kong also expressed concern.

"We urge China to respect Hong Kong's high degree of autonomy consistent with the principle of 'one country, two systems'," US consulate spokeswoman Kristin Haworth told Reuters.

The Chinese Embassy in London, in an email reply to Reuters late on Thursday, did not respond directly to Rogers' allegation that it had warned him through a middleman that he could be refused entry to Hong Kong.

"It is a matter of Chinese sovereignty whether or not to allow entry of any foreigners," it said, echoing the foreign ministry.