London has warned dual Chinese-British nationals they may not get consular assistance in Hong Kong even if they enter the city on their British passport.

The warning came after the British government said it had been told Hong Kong does not recognise dual nationality.

Updated travel advice posted on the Foreign, Commonwealth and Development Office website on Monday said the British consulate had been told “that Hong Kong, like other parts of China, does not recognise dual nationality”.

“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. If this is the case, the British consulate may not be able to offer you consular assistance.”

The update advised those who had “formally renounced Chinese citizenship” to “carry evidence” when they travelled to Hong Kong or mainland China.

Canada’s foreign affairs department reported last week that a dual-national in prison in Hong Kong had been required to make a declaration of nationality last month.

“We are aware of more such incidents involving dual nationals of other countries,” a Global Affairs Canada representative said at the time.

Washington expressed “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”.

The British consulate also said London was “seeking answers” from their counterparts in Hong Kong.

It is not uncommon for Hongkongers to hold multiple passports, but Chinese nationality law does not recognise dual nationality, and authorities may refuse to grant consular access to those who retain Hong Kong or Chinese passports.

Advice published by the British government warned that those people who were British and Chinese were “likely to be regarded by the Chinese authorities as a Chinese citizen, even if you travel to China on a British passport”.

It said the best option was to “only travel to China on your British passport, with a Chinese visa in it”.

On Monday, the Hong Kong Security Bureau said China’s nationality law had been applied in the city since the 1997 handover.

In a statement, the bureau pointed to an explanation from the country’s top decision-making body, the National People’s Congress Standing Committee, in 1996, which clarified the nationality law and said a person was considered Chinese if they were born “in the Chinese territories, including Hong Kong, or where a person satisfies the criteria laid down in [China’s nationality law] for having Chinese nationality”.

The bureau added that unless a person had an application to renounce their nationality approved by the Hong Kong Immigration Department, they were still considered Chinese.

“Chinese nationals of the [Hong Kong special administrative region] with right of abode in foreign countries may, for the purpose of travelling to other countries and territories, use the relevant documents issued by the foreign government,” the bureau said.

“However, they will not be entitled to consular protection in [Hong Kong] and other parts of [China] on account of their holding the above-mentioned documents.”

The bureau’s statement largely echoed the information found on the Immigration Department’s website.

Professor Lau Siu-kai, vice-chairman of Beijing’s semi-official think-tank the Chinese Association of Hong Kong and Macau Studies, said the travel alert was “more like a reminder or disclaimer”.

“More foreign countries are getting concerned about the nationality issue because of the BN(O) dispute and the implementation of the national security law,” he said.