Up to three million residents of the former British colony have been offered the right to settle in the UK and ultimately apply for citizenship after Beijing imposed a controversial national security law.

The UK government believes the new legislation breaches the Sino-British Joint Declaration, which aimed to smooth the transition when the territory was handed back to China in 1997.

Dominic Raab said that if Beijing tried to stop people with British National (Overseas) status from leaving Hong Kong, there would be little that could be done by the UK.

And the Chinese Embassy in the UK said Beijing reserves the right to take "corresponding measures" and urged the UK against "interfering in Hong Kong affairs in any way".

Australia is also considering offering visas to Hongkongers found to be in danger following the new law.

Prime Minister Scott Morrison told reporters safe haven arrangements were being looked at "very actively", adding: "Are we prepared to step up and provide support?' The answer is, 'yes'."

Raab told ITV: "Ultimately if they follow through on something like that there would be little that we could do to coercively force them.

"There is an issue around freedom and human rights in Hong Kong, and there is an issue around China keeping its word on an international obligation it made to the United Kingdom back in 1984.

"I wouldn't want to be naive about this: I think we need to be realistic. But I do think that China as a rising, leading member of the international community is sensitive to the reputational risk in all of this but clearly not sufficiently that it hasn't proceeded anyway.

"There is diplomatic leverage, there are other ways that we can persuade China not to fully implement either the national security law or some of the reprisals you talk about.

"But ultimately we need to be honest that we wouldn't be able to force China to allow BN(O)s to come to the UK."

As of February, there were nearly 350,000 BN(O) passport holders, while the government estimates there are around 2.9 million BN(O)s living in Hong Kong.

Asked how many people the government anticipated would come to the UK from Hong Kong, he said: "It's difficult to give a precise forecast but I think it is fair to assume that only a proportion of those that are offered the new status that we set out in the House of Commons today will take it up.

"I think the majority of people will probably hunker down in Hong Kong and others would move to other countries in the region."

The security law in Hong Kong - which came into effect on Tuesday night - makes activities deemed subversive or secessionist punishable by imprisonment, and is seen as targeting anti-government demonstrators.

Australian Associated Press