China has lodged solemn representations to Britain over its foreign secretary's remarks on Hong Kong affairs, Foreign Ministry spokeswoman Hua Chunying said on Thursday, after Hong Kong barred the entry of a British activist.

Hua reiterates that the central government had a responsibility for foreign matters related to Hong Kong.

"The Chinese central government and the special administrative region government handle the relevant issue in accordance with the law," Hua said. "Allowing or not allowing people in is China's sovereignty ... Hong Kong affairs are a purely internal matter for China."

Benedict Rogers, a British human rights activist and co-founder of the Conservative Party's Human Rights Commission, was denied entry to Hong Kong on Wednesday.

British Foreign Secretary Boris Johnson said he was "very concerned" that Rogers was denied entry to the city and demanded an "urgent explanation" from China.

Hua said China had lodged a solemn representation – an official complaint – with Britain over Boris Johnson's comment.

When asked about the incident on a radio talk show, Hong Kong's Chief Executive Carrie Lam said, "Ultimately, under the Basic Law, the central government is responsible for foreign affairs."

Asked if immigration controls fell under Hong Kong jurisdiction, Lam said: "It has to be considered whether foreign affairs are involved during the process of immigration.

"If you say everything falls under Hong Kong's high degree of autonomy, this is not what the Basic Law says."

Hong Kong's Immigration Department said it does not comment on individual cases.