The Foreign Office has summoned the Chinese ambassador to make clear the UK's "deep concern" over the new Hong Kong national security law.
Liu Xiaoming was called to a meeting with the Foreign Office's permanent under-secretary Sir Simon McDonald on Wednesday, hours after China imposed the legislation on the city, Sky News reported.
Sir Simon made clear the UK's "deep concern" over the new law, which was proposed a month ago and came into effect in Hong Kong on Monday.
It is only the second time a Chinese ambassador has been called to the Foreign Office about Hong Kong since 1984.
Britain's Foreign Office was not immediately available for comment.