The Home Office appears to have dramatically widened the pool of Hong Kong citizens that will be eligible to apply for UK citizenship, implying millions may be able to apply if China presses ahead with plans for draconian new security legislation in the territory. The UK government’s decision has infuriated the Chinese government, and could risk a backlash among traditional Conservative voters opposed to immigration. On Thursday the foreign secretary, Dominic Raab, said the UK would extend visa rights for about 350,000 British national (overseas) (BNOs) passport holders if Beijing went ahead with its plans to impose a national security law on Hong Kong. He said visa rights would be extended from a period of six months to an extendable 12 months and so provide an unspecified pathway to future citizenship. However, on Friday, the Home Office in a blog clarified that the rights would cover anyone eligible to apply for a BNO passport currently living in Hong Kong, of which there are an estimated 2.9 million people. Most of the additional 2.55 million people have held a BNO passport in the past but not renewed it. The BNO was issued to Hong Kong residents born before the handover of the territory from UK to Chinese sovereignty in 1997. The Home Office blog said all those applying for extendable visas would be subject to normal immigration laws, a phrase on which it did not expand. The blog added further details will be made available. Beijing adopted a resolution on Thursday that opens the way for a new national security law in the former colony. The resolution has been condemned by several western governments, and a joint British-American effort to raise the issue at the UN security council in New York was pushed back by an alliance led by Russia and China. Priti Patel, the UK home secretary, said she was deeply concerned at China’s proposals for legislation. The new law bans treason, subversion and sedition in Hong Kong and follows months of anti-government demonstrations last year. Patel said: “If China imposes this law, we will explore options to allow British nationals overseas to apply for leave to stay in the UK, including a path to citizenship.” She said the UK would continue to defend the rights and freedoms of the people of Hong Kong. The Home Office said on its website that the 12-month period could be extendable for eligible parties. The Chinese foreign ministry said London had agreed the passport holders should not enjoy residency rights and that the offer violated international law, while warning of retaliation. “China reserves the right to take necessary measures,” ministry spokesman Zhao Lijian said, adding Beijing considered Chinese residents in Hong Kong to be Chinese citizens, regardless of which passport they held. China would fear mass migration from the prosperous international city, and believes a much wider offer could destabilise Hong Kong’s economy. It is not clear if Conservative MPs will be prepared to see hundreds of thousands of Hong Kong citizens come to the UK so soon after the party fought a long battle to control immigration by leaving the European Union. On Friday, Bob Seeley, one of the lead campaigning Tory MPs on the issue, wrote: “The possibility of a mass flight from Hong Kong may become one of those rare occasions where mass asylum in the UK is morally right, as it was with ethnic Indians in Uganda expelled by that country’s insane dictator, Idi Amin, and accepted by the UK and others in 1972.”