The UK has warned Britons in Hong Kong that the territory has stopped recognising dual nationality and it might no longer be able to help citizens who also hold a local passport.

“If you have both British and Chinese nationality you may be treated as a Chinese citizen by local authorities, even if you enter Hong Kong on your British passport,” the UK’s Foreign, Commonwealth and Development Office said in an update to its travel advice. “If this is the case, the British consulate-general may not be able to offer you consular assistance.”

The shift in practice is in line with other efforts to bring the territory — which was promised a high degree of autonomy from the Chinese mainland for decades in the 1997 handover from the UK — closer to the rest of China.

It also coincides with the crackdown by Hong Kong authorities, which began with the promulgation of a new tough national security law in the middle of last year and has now reached beyond jailing opposition leaders to reforming Hong Kong society. The law was Beijing’s response to persistent pro-democracy protests in 2019.

The warning highlights how life has become more uncertain for the hundreds of thousands of dual Chinese nationals in Hong Kong, who have been able to work in the city while maintaining family links overseas and vice versa.

The British warning was issued following a move by the Hong Kong government to halt diplomats, including those from Canada and the US, from offering assistance to Chinese dual citizens with a foreign passport who had been arrested or detained in the city in certain cases.

Dual nationality is not recognised in Chinese nationality law, which also applies in Hong Kong, but the Hong Kong government has long allowed diplomats access to these dual citizens.

“If there might be problems accessing these individuals now . . . there might be a problem gaining access in the future in hypothetical cases,” one diplomat said.

Hong Kong’s security bureau said locals with international travel documents were not entitled to consular protection, and needed to have their application to renounce their Chinese citizenship approved before being considered a foreign national.

China signalled last month it would make life more difficult for Hong Kongers with a passport the UK offers to some of the territory’s residents. The move was in response to Britain offering to up to 3m of the city’s population a new path to citizenship via these British National Overseas passports.

The UK’s Home Office has said that about 7,000 BNO passport holders were granted leave to remain in the UK between last July and January, even before the new citizenship route formally opened to applications.

Beijing said it would halt recognising BNO status passports as valid identification, while the local authorities in Hong Kong said they would require airlines to check that Hong Kong residents boarding flights had either a local passport or identification.

At this stage, the steps have not affected a vast number of locals as most do not rely on these passports to travel, but Beijing has said it reserved the right to take further measures.

A Foreign Office spokesperson said: “It is wrong that Hong Kong authorities are withdrawing our consular access to British dual nationals in prison and preventing us providing support we've given since 1997. Alongside international partners we urge the authorities to restore access to our nationals immediately.”

The US and Canada have both voiced concern in recent weeks about restrictions on access to detained dual nationals and said Hong Kong was forcing people to make a decision about which nationality they were.

“We have deep concerns that this new Hong Kong policy will compel people to declare their citizenship under duress and without an opportunity to understand the full implications of the declaration, and is ripe for abuse by law enforcement,” the US State Department said.

The department added that consular notification and access obligations towards dual nationals applied under the Vienna convention on consular relations.

Johnny Patterson, policy director of Hong Kong Watch, a London-based charity campaigning on human rights in Hong Kong, said the city had traditionally been a “melting pot” and that many of its residents were dual nationals.

“The decision to stop recognising dual nationality is self-defeating,” he said. “It will fail to stop people leaving under the BNO scheme but may add to the chilling effect which is already threatening Hong Kong’s reputation as an international hub.”