China’s foreign ministry has accused Britain of “gross interference” in the country’s affairs after Boris Johnson said he would offer millions of Hong Kong residents a path to UK citizenship if Beijing pushed ahead with a controversial security law for the city. The ministry’s spokesman Zhao Lijian told Britain to “step back … otherwise there will be consequences” and said China had made “serious representations” to London over its offer to holders of British national (overseas) passports. Although pro-democracy politicians and protesters in Hong Kong welcomed Britain’s offer, most said they would prefer international efforts to focus also on protection for the city – in the form perhaps of sanctions against China or officials in Beijing – and not just on providing sanctuary for those who want to flee. “I think it’s a shame in a way that they only offer us an exit, and do not offer to stand by us in our fight for Hong Kong,” said Lee Cheuk Yan, a veteran activist who was arrested in a widely condemned crackdown by Hong Kong authorities this year. “I can understand some people are so fearful they want to leave,” Lee said. “[But] what about those who stay behind? Are you going to give us up?” There are also concerns that the offer excludes the city’s youth, who have been at the forefront of the year-long protest movement, because the BNO passports are issued only to people born before the 1997 end of British colonial rule. “We welcome the British giving BNO holders the right of abode. But young people born after British left are not BNO holders. They’re our hope and should be given the priority over old people like me who are near the end of their lives,” said Jimmy Lai, a media tycoon and outspoken critic of Beijing who was arrested this year. Britain’s position is based on the joint declaration between the two countries, which paved the way for the handover of Hong Kong in 1997 and promised the city 50 years of autonomy from that date, with its rights and freedoms protected. The agreement was registered with the UN, and the UK considers it a binding international treaty; Beijing now argues that as soon as the handover was complete, it effectively became void. “There is no single word or clause in the Sino-British joint declaration that entitles the UK to any responsibility for Hong Kong after its return,” Zhao said on Wednesday at a regular press briefing. “The UK has no sovereignty, governance or supervision over Hong Kong … Therefore, the UK has no right to lecture or interfere in China’s internal affairs under the pretence of the joint declaration.” Johnson said Britain would activate its offer only if Beijing “pushes ahead” with the law. China’s rubber-stamp parliament has passed the law but details of how it will be implemented in Hong Kong have not yet been spelled out. On Wednesday the foreign secretary Dominic Raab said the UK was prepared to pay the price of a free trade deal with China. “Us extending the UK BNO passport holder offer to come to the UK in the way that I have described is not contingent on a free trade deal or anything like that,” he told Sky News. “We wouldn’t allow that to get in the way of us living up to our responsibilities as a matter of principle both for moral reasons and international standing.” About 350,000 people hold British national (overseas) passports, and another 2.5 million people could become eligible for one under Britain’s offer. There is still a lot of ambiguity about what the “path to citizenship” proposed by Johnson would look like and who exactly might be eligible. Hong Kong’s leader, Carrie Lam, accused foreign critics of “blatant double standards”, claiming every country had the right to pass national security laws. “Why should they object, resist or even condemn and take sanctions against Hong Kong and the People’s Republic of China for taking similar actions to protect (themselves),” she told reporters. For now though, many in her own city do not believe those assurances. Elaine Leong, at 42 does not want to leave Hong Kong but plans to apply for a British national (overseas) passport for the first time, “just in case”. “It is more secure to have a foreign passport in our hands in case anything drastic happens. It’s not for us, but for our child’s future,” she said. “But having said that, we don’t have any roots in Britain, we don’t know anyone. How easy is it to find a job when the economy in the UK isn’t good? And in a foreign country, we’d be treated like second-class citizens. It’s not our home.” Verna Yu contributed reporting