The threat from Beijing came as Boris Johnson announced he was ready to open the door to nearly three million Hong Kong citizens if China imposed a new security law critics say would rob the territory of its autonomy. The Prime Minister has offered to make what he says would be one of the "biggest changes" in the history of the British visa system to allow 2.85 million Hong Kong citizens the chance of fully-fledged citizenship.
Many people in Hong Kong fear their way of life, which China pledged to uphold, is under threat
The move represents a dramatic escalation in the stand-off between the UK and China over Hong Kong.
China's ceremonial legislature, the National People's Congress, endorsed a security law for Hong Kong last week.
The law will alter the territory's mini-constitution, or Basic Law, to require its government to enforce measures to be decided later by Chinese leaders.
Critics say the law erodes the "one country, two systems" framework that promised Hong Kong freedoms not found in mainland China for 50 years.
Protesters have taken to the streets in Hong Kong to demonstrate , despite coronavirus social distancing restrictions still being in place.
Mr Johnson said the national security law would breach the treaty between China and the UK and would "dramatically erode" Hong Kong's autonomy.
He said: "If China imposes its national security law, the British Government will change our immigration rules and allow any holder of these passports from Hong Kong to come to the UK for a renewable period of 12 months and be given further immigration rights, including the right to work, which could place them on a route to citizenship."
He continued: "This would amount to one of the biggest changes in our visa system in British history.
"Many people in Hong Kong fear their way of life, which China pledged to uphold, is under threat.
"If China proceeds to justify their fears, then Britain could not in good conscience shrug our shoulders and walk away; instead we will honour our obligations and provide an alternative."