China may not recognise Hongkongers’ British nationality acquired through the United Kingdom’s newly proposed citizenship scheme, according to a senior Hong Kong adviser to Beijing.

London’s latest offer to extend a broader path to citizenship for 3 million Hongkongers who either have a British National (Overseas) – BN(O) – passport or are eligible for one – in response to China’s sweeping national security law was met with Beijing’s warning of unspecified “corresponding measures”.

The tough new national security law took effect for the former British colony at 11pm on Tuesday.

Lau Siu-kai, vice-president of the semi-official Chinese Association of Hong Kong and Macau Studies, said Beijing would most likely “retaliate” by not recognising British citizenship acquired through this scheme.

“If a person would like to be registered as a British national in Hong Kong, he or she should be asked whether or not that nationality was acquired through this scheme. If so, the person would remain legally as a Chinese national,” Lau said.

“Of course, there are difficulties in implementation as to how to verify the way that one acquired the British nationality,” Lau said. “Yet, the Hong Kong government can hold people legally accountable by making them declare it on paper.”

Under the latest proposal announced by British Prime Minister Boris Johnson on Thursday, BN(O)-eligible Hongkongers will be allowed to stay in Britain for five years and ultimately be eligible to apply for citizenship.

The new law that targets secession, subversion, terrorism and collusion with foreign forces in the Hong Kong has prompted Britain and Australia to suggest making a “haven” for Hongkongers who wanted to leave.

However, if Beijing announced it did not recognise the British citizenship, as Lau suggested, Hongkongers who successfully gained British citizenship through this scheme would not be able to declare their British nationality on returning to Hong Kong. This would mean legally remaining a Chinese national within Chinese borders and not being able to receive British consular protection.

China does not recognise dual nationality or citizenship. Hongkongers are subjected to some leeway according to the official document of “explanations” made by the Standing Committee of the National People’s Congress released on May 15, 1996, before the handover on 1997.

According to the official NPC document, Beijing does not bar Hongkongers who are holders of dual nationality to use other passports for travel purposes, but they are not entitled to seek consular protection from their second country in the city or other parts of the country. In other words, these people would only be able to seek diplomatic protection from their non-Chinese country of choice in Hong Kong if they successfully renounced their Chinese nationality and declared their other nationality.

Yet, this condition does not apply to a small number of Hongkongers who hold dual Hong Kong-British nationality from the “British Nationality Selection Scheme” (BNSS) before the handover, as stated in the NPC explanation: “the British citizenship acquired by Chinese nationals in Hong Kong through the ‘British Nationality Selection Scheme’ will not be recognised. They are still Chinese nationals and will not be entitled to British consular protection in the Hong Kong Special Administrative Region and other parts of the People’s Republic of China.”

Under the existing procedure to apply for a change in nationality, the applicant would be required to declare: “My British citizenship is not acquired through the British Nationality Selection Scheme.” Hong Kong Immigration will not proceed with the application if such a declaration is not made.

Lau said the BN(O) passport holders who were eligible for British citizenship would be treated the same and made to make similar declarations if they sought to renounce Chinese citizenship in future.

The BNSS was London’s only offer of nationality to 50,000 Hongkongers and their family before the city returned to Chinese rule in 1997. The decision was met with protests in Hong Kong at that time as other Hongkongers were only granted the British National (Overseas) passport that did not come with the right of abode, and was generally perceived as no more than a travel document.

China’s treatment of Hong Kong-British dual nationals was made especially clear in 2016 when bookseller Lee Bo, a British passport holder, went missing and was later found to be under questioning by Chinese authorities.

London expressed concern about Lee’s situation, but Chinese Foreign Minister Wang Yi said Lee was “first and foremost a Chinese citizen”.

Craig Choy Ki, a lawyer and a spokesman for campaign group Equal Rights for BN(O) passport holders, said he did not think Beijing’s rejection of recognising the BNSS had significant practical impacts on those Hong Kong-British dual nationals. Yet, he said, such a policy would have implications for those who could be charged with national security-related offences in the future.

“When looking at Lee Bo's case, we can see that these Hong Kong-British dual nationals will only be considered as Chinese nationals and be subjected to laws that apply to Chinese nationals only. For example, be put under charges of treason,” Choy said.

Hong Kong Chief Executive Carrie Lam Cheng Yuet-ngor, her husband and her two sons successfully gained British citizenship through the BNSS, but Lam said she gave it up in 2007 when she was appointed secretary for development.

A researcher on mainland-Hong Kong affairs who did not want to be named said the heated discussion about nationality stirred up by the imposition of the national security law was irrelevant.

According to Article 38 of the national security law, it covers offences by non-permanent residents of Hong Kong outside the city. The law also applies to Hong Kong permanent residents, organisations and companies set up in Hong Kong, even when an offence is committed outside the city.

“Whether or not that person has a dual nationality or not does not matter, if he or she has breached the national security law, the person will be subjected to the offences,” the mainland-based researcher said.