China tells UK and others to stop 'smearing' new Hong Kong security law

China urged Britain to stop making "groundless accusations" about Hong Kong's new national security law and denounced countries that "smear" legislation that critics say further threatens freedoms in the former British colony.

Hong Kong lawmakers unanimously passed the law on Tuesday, fast-tracking legislation to fortify perceived threats and cracking down on dissent after sometimes violent pro-democracy protests swept the territory in 2019.

Hong Kong affairs are "purely China's internal affairs" and the "British side is not qualified to make irresponsible remarks", the Chinese Embassy in Britain said in a statement on Wednesday.

British Foreign Secretary David Cameron said the law will have far-reaching implications and further damage the rights and freedoms of Hong Kong people. He said it undermined the terms of the 1984 agreement under which Britain handed back its colony to China in 1997.

The United States said it believes the passing of the new bill has the potential to accelerate the closing of a once-open society.

"China expresses strong dissatisfaction and firm opposition to some countries and institutions that slander and smear" the legislation, said Lin Jian, Chinese foreign ministry spokesperson when addressing a question on the law at a regular daily press briefing.

"The firm determination to oppose any external interference in Hong Kong affairs is unswerving. Any attack on the protection of national security is a smear. It will never succeed, will be doomed to failure," Lin said.

Hong Kong returned to Chinese rule with the guarantee its freedoms, including freedom of speech, would be protected under a "one country, two systems" formula. Critics of a 2020 national security law say those freedoms have eroded swiftly.