Holders of British National (Overseas) status - a legacy of United Kingdom rule over Hong Kong up to 1997 - will from Sunday be able to apply to live and work in the United Kingdom for up to five years, and eventually seek citizenship.
Britain's government on Friday vowed to stand by the people of its former colony Hong Kong against a Chinese crackdown as it prepared to launch a new visa scheme potentially benefiting millions.
But China hit back by saying it would not recognise the BNO passport as a valid travel document from January 31.
Britain moved to open its doors after China imposed a sweeping national security law on Hong Kong, sharply curtailing political rights, which London said was a clear breach of the handover agreement.
Ignoring the fact that Hong Kong has returned to China for 24 years and China's solemn position, the British side blatantly violated its commitment and insisted on issuing the so-called "tailored policy" for Hong Kong residents with BNO passports to stay and naturalize in the country, and has repeatedly expanded the scope of application, Zhao said.
About 300,000 people are expected to leave Hong Kong for Britain using the route.
But a BN (O) passport is available to a huge number of people - about 70 percent of Hong Kong's total population of 7.5 million.
The prime minister's official spokesman said: "We have been clear that we will not look the other way when it comes to Hong Kong people with British National (Overseas) status who now have a choice to come and live and work and study in the UK".
The new procedure honours the "profound ties of history and friendship" between the province and Britain, Johnson said in a statement.
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"China's imposition of the National Security Law in Hong Kong constitutes a clear and serious breach of the (pre-handover) Sino-British Joint Declaration contrary to global law".
To enter mainland China, they need to use their Hong Kong passport.
"It's the beginning of the end for people in Hong Kong with Canadian status", said Vancouver immigration lawyer Richard Kurland.
China's reaction came as Britain vowed to stick with the earlier announced plan to admit Hong Kongers who have had their rights and freedoms restricted following China's imposition of the National Security Law in the former British colony. A BNO passport holder in Hong Kong could still use their Hong Kong passport or identity card.
Now it has become one of the few ways out for Hong Kongers hoping to start a new life overseas, as authorities conduct mass arrests against democracy supporters and move to purge the restless city of dissenting views.
In October, the British government estimated that between 123,000 and 153,700 BN (O) passport holders and their dependents could arrive in the first year of the migration offer, and up to 320,000 over five years.
"The worst-case scenario would definitely be the Chinese government urging all Hong Kong [people] to renounce the BN (O) otherwise there will be consequences", said Nathan Law, an activist who left Hong Kong for Britain.
China has said the law is necessary to prevent the type of protests seen in Hong Kong during much of 2019.