British Prime Minister Boris Johnson said on Tuesday he would offer millions of Hong Kong residents visas and a possible route to U.K. citizenship if China persists with its national security law.

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“Many people in Hong Kong fear their way of life — which China pledged to uphold — is under threat,” he wrote in an article for The Times newspaper and the South China Morning Post.

“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.”

‘Route to citizenship’

About 3,50,000 people in Hong Kong currently hold British National (Overseas) passports, which allow visa-free access to Britain for up to six months, Mr. Johnson wrote.

Another 2.5 million people would be eligible to apply for one.

“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 U.K. 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 wrote.

The new law was brought in after a wave of pro-democracy protests in Hong Kong, and approved by China’s Parliament as necessary to tackling “terrorism” and “separatism”.

Opponents fear it will lead to political oppression in the financial hub, eroding freedoms and autonomy supposedly guaranteed in the 1997 handover from Britain to China.

Mr. Johnson said the Hong Kong law would “curtail its freedoms and dramatically erode its autonomy”. If implemented, “Britain would then have no choice but to uphold our profound ties of history and friendship with the people of Hong Kong”, he wrote.

British Foreign Secretary Dominic Raab has further angered Beijing by suggesting that it had time to “reconsider” the plan.

Chinese Foreign Ministry spokesman Zhao Lijian said Beijing had lodged “serious representations” with London. “We advise the U.K. to step back from the brink, abandon their Cold War mentality and colonial mindset, and recognise and respect the fact that Hong Kong has returned” to China, Mr. Zhao said.

Mr. Zhao said London must “stop interfering in Hong Kong’s affairs and China’s internal affairs, or this will definitely backfire.