China has said it reserves "the right to take corresponding measures" if the UK pushes forward with its plan to give three million Hong Kong residents the right to settle in the country.
It comes after Boris Johnson accused Beijing of a "clear and serious breach" of its treaty with Britain by imposing a much-criticised national security law on the territory.
The law makes secessionist, subversive, or terrorist activities illegal, as well as foreign intervention in the city's internal affairs.
The Prime Minister said he would introduce a route for people with British National (Overseas) (BNO) status to apply for visas to live and work in the UK and apply for citizenship.
In response, the Chinese Embassy in the UK said such a move would be in breach of "international law and basic norms governing international relations".
A statement said: "We firmly oppose this and reserve the right to take corresponding measures."
It continued: "We urge the British side to view objectively and fairly the national security legislation for Hong Kong, respect China’s position and concerns, refrain from interfering in Hong Kong affairs in any way."
On Wednesday, Foreign Secretary Dominic Raab accused Beijing of a "grave and deeply disturbing" breach of the treaty which "threatens the strangulation" of Hong Kong's freedoms.
He told MPs the "bespoke" new arrangement to be implemented in the coming months would grant BNOs five years' limited leave to remain in the UK with the ability to live and work.
They would then be eligible to apply for settled status and would be able to apply for citizenship after 12 months with that status.
As of February, there were nearly 350,000 BNO passport holders, while the government estimates there are around 2.9 million BNOs living in Hong Kong.