LONDON — Prime Minister Boris Johnson raised the stakes in a brewing confrontation with China on Wednesday, promising to allow nearly 3 million people from Hong Kong to live and work in Britain if Beijing moves forward with a new national security law on the former British colony.

Johnson’s offer, made in a column in The Times of London, opens the door to a significant influx of people fleeing Hong Kong, should the situation in the territory deteriorate further. But it leaves unanswered thorny questions about how difficult it would be for these arrivals to obtain British citizenship.

Describing it as one of the biggest changes in visa regulations in British history, Johnson said the roughly 350,000 Hong Kong residents who hold a British overseas passport, as well as some 2.5 million who are eligible to apply for one, would be granted 12-month renewable visas that would allow them to work in Britain and put them on a path to citizenship.

“Many people in Hong Kong fear that their way of life — which China pledged to uphold — is under threat,” Johnson wrote. “If China proceeds to justify their fears, then Britain could not in good conscience shrug our shoulders and walk away; instead we will honor our obligations and provide an alternative.”

Johnson’s offer applies to Hong Kong residents whose passports bear the insignia of the British government — reflecting their status in the territory before it returned to Chinese sovereignty in 1997 — but do not confer the rights of citizenship that come with an ordinary British passport.

China has angrily rejected the idea, declaring Britain has no right to make such an offer to Hong Kong residents who are Chinese nationals.

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There is also the question of what Britain will do if the Chinese government refuses to let people leave.

“With the Chinese government under Xi Jinping, we cannot rule out the possibility that they won’t be allowed to leave,” said Steve Tsang, director of the SOAS China Institute in London, referring to China’s leader.

“We don’t have gunboats to send to Hong Kong to get them out of there.”