"The UK now considers Beijing to be in a state of ongoing non-compliance with the Sino-British Joint Declaration," the foreign ministry said in a statement.

The treaty was signed before Britain handed Hong Kong back to China in 1997 and was designed to allay fears about its future under Beijing's rule.

It guarantees the financial hub special status including a high degree of autonomy to manage its own affairs and the right to freedom of speech.

But British Foreign Secretary Dominic Raab said that Beijing's decision "to impose radical changes to restrict participation in Hong Kong's electoral system" was a "further clear breach" of the agreement.

"This is part of a pattern designed to harass and stifle all voices critical of China's policies and is the third breach of the Joint Declaration in less than nine months," he said.

"I must now report that the UK considers Beijing to be in a state of ongoing non-compliance with the Joint Declaration," he added, further ramping up tensions between the two nations.

Britain has been a strong critic of China's crackdown on pro-democracy campaigners in Hong Kong, and angered Beijing by announcing a new visa scheme offering millions of its residents a pathway to British citizenship.

The system went live in January as the city's former colonial master opened its doors to those wanting to escape China's crackdown on dissent.

- 'Growing gulf' -

Beijing has acted decisively to dismantle Hong Kong's democratic pillars after massive protests there in 2019.

China's rubber-stamp parliament on Thursday voted to give Beijing the power to veto candidates as it moves to ensure that only "patriots" run the city.

Raab said the latest move was a "demonstration of the growing gulf between Beijing's promises and its actions.

"The UK will continue to stand up for the people of Hong Kong," he added.

"China must act in accordance with its legal obligations and respect fundamental rights and freedoms in Hong Kong."

The new election rules sparked international condemnation.

United States Secretary of State Antony Blinken called it "a direct attack on autonomy promised to people in Hong Kong under the Sino-British Joint Declaration".

"These actions deny Hong Kongers a voice in their own governance by limiting political participation, reducing democratic representation and stifling political debate," Blinken said.

The European Union warned China it could take "additional steps" as it condemned the vote.

"The National People's Congress of the People's Republic of China adopted today a decision that will have a significant impact on democratic accountability and political pluralism in Hong Kong," EU foreign policy chief Josep Borrell said on behalf of the 27-nation bloc.

Brussels has previously decided to limit exports of equipment that could be used for surveillance in Hong Kong and EU foreign ministers have discussed the possibility of broader sanctions if the situation worsens.