"The enactment and imposition of this national security law constitute a clear and serious breach of the Sino-British Joint Declaration," Prime Minister Boris Johnson said in parliament.

"It violates Hong Kong's high degree of autonomy and is in direct conflict with Hong Kong's basic law. The law also threatens the freedoms and rights protected by the joint declaration," he said.

"We made clear ... that if China continued down this path we would introduce a new route for those with British National Overseas Status to enter the UK, granting them limited leave to remain, with the ability to live and work in the UK and thereafter to apply for citizenship and that is precisely what we will do now."

Meanwhile, British Foreign Secretary Dominic Raab said he would set out shortly the action Britain would take with its international partners.

"China, through this national security legislation, is not living up to its promises to the people of Hong Kong," he said. "We will live up to our promises."

Asked about how the West should deal with President Xi Jinping, Raab said:

"Obviously, China is a leading member of the international community. And it is precisely because of that, that we expect it to live up to its international obligations and its international responsibilities, for trust in China's ability to do that, today has been a big step backwards."