The intervention is likely to worsen relations between the UK and China with Beijing saying the plans “violate international law”.
Boris Johnson has pledged to allow nearly three million Hong Kong citizens the right to live and work indefinitely in the UK, with a route to full citizenship, if China imposes a controversial new national security law on the city.
The prime minister described the potential move as “one of the biggest changes in our visa system in history”.
Previously, Foreign Secretary Dominic Raab had only said the offer would be available to the 350,000 current holders of a British National Overseas passport in Hong Kong and their dependants.
However, Mr Johnson has said he would extend the same offer to a further 2.5 million people in Hong Kong who do not hold a BNO passport but are eligible for one.
Last month, China announced plans to directly impose a new national security law in Hong Kong.
The detail of the legislation is currently being drafted but, under the current proposals, many fear the law could be used to diminish Hong Kong’s traditional freedoms of the press, of speech and of protest, among others.