A new British policy allowing Hong Kong residents to claim British citizenship is a violation of international law and interferes with China's internal affairs, China's embassy in London said on Thursday.

British Interior Minister Priti Patel said on Wednesday that Hong Kong people with British National Overseas visas would be able to apply for citizenship starting from January 2021.

Britain had made that decision despite opposition from Beijing and China would respond strongly if it was not reversed, the Chinese embassy said in a statement.

The move "severely violated (Britain's) own commitments, seriously interfered with the internal affairs of China and seriously violated international law and the basic norms of international relations," it said.

London's decision, which could allow nearly three million Hong Kong residents to settle in Britain, came after Beijing imposed a national security law on the former British colony that democracy activists said would end the freedoms promised in 1997 when the territory was returned to Chinese rule.

Britain says the law breaches the terms of the handover treaty agreed in 1984. China accuses Britain of interfering in Hong Kong and Chinese affairs.

"The Chinese side urges the British side to recognise the reality that Hong Kong has returned to China, to look at the Hong Kong national security law objectively and immediately correct its mistakes," the embassy said.