Young Hongkongers may not be given an automatic right to move to the UK and will need to prove they still live with their parents in order to do so.

More details of the Home Office’s immigration plans to help grant rights to British National (Overseas) (BNO) citizens who live in Hong Kong so they can come to live and work in the UK were announced on Wednesday.

The news comes as the UK suspended its extradition treaty with Hong Kong and slapped an arms embargo on the territory in response to China’s national security law.

But questions have remained about how the immigration measures can protect the rights of 18 to 23-year-olds, who are too young to have been able to register for BNO status like their parents.

Campaigners have previously warned the measures could exclude youngsters, who are more likely to be the target of authorities as they have been most active in demonstrations.

UK-based charity Hong Kong Watch hailed the plans as a “step in the right direction” but warned the limitations being placed on young people were “an issue of some concern”, adding that it was “critical” the immigration route did not turn into an economic policy for the privileged.

Officials said the arrangements will prevent families being split up and allow BNOs to come to the UK with their spouses, children under the age of 18 and any adult children they have, as long as they are still a “family unit”. For example, if they still live with their parents or are dependants in some form.

The Government may grant permission for other cases under exceptional circumstances – such as for elderly parents of BNOs who need care.

Last week, Home Secretary Priti Patel sought to give assurances to MPs raising questions about the concerns over youngsters, saying that she was “actively looking at that particular age group and cohort” and considering a “number of options”.

On Wednesday, Foreign Secretary Dominic Raab said: “Today’s announcement shows the UK is keeping its word: we will not look the other way on Hong Kong, and we will not duck our historic responsibilities to its people.”

Around three million people are thought to be eligible for BNO status and there are around 366,000 passports in circulation.

Those born after 1997 are too young to have been able to register, but some will be too old to be considered a child under the UK immigration system.

The Home Office is still trying to establish how many people it expects could come to the UK under the new immigration rules.

From January, BNOs and their immediate family can apply for 30-month or five-year visas to live, work and study in the UK and can seek British citizenship once they have been in the country for more than five years.

They will not need a job or a valid passport to apply. They can use an expired passport to provide their identity or the Home Office has pledged to help confirm the status of those without documentation.

Although applicants will be able to access healthcare and go to school, they will not initially have access to benefits and will need to prove they can support themselves financially for the first six months of their stay.

They also have to provide a tuberculosis test certificate and show a commitment to speaking English.

Criminal history will be taken into account but it is unclear how immigration officials will view convictions for participating in protests, which constitute breaking China’s national security laws.

Applicants will have to pay for the visa and the immigration health surcharge, but the cost of the fees are still being decided. If they wish to remain in the country permanently they will need to later pay a settlement fee of around £2,400.

It is unclear what support could be offered for applicants with financial difficulties.

While the upfront costs of the 30-month visa will be cheaper, it would have to be resubmitted for those keen to stay in the country after it expires.

Before January, people with a valid Hong Kong passport can turn up at the UK border and will be granted entry for six months. They can then apply for the new visas or any others as part of the immigration system where they meet the criteria.

This means young Hongkongers may be able to enter the UK without their BNO parents and then apply for the youth mobility scheme or as skilled workers under the forthcoming points-based system instead, if eligible.

Questions still remain over how the rights of BNOs will be protected if China resists the measures, and whether there are fears Chinese agents could seek to exploit the new immigration route to the UK.

While the Government cannot force China into allowing Hong Kong citizens to leave, it is expected to live up to its international obligations, a spokesman said.