Undocumented migrants in Britain will still be able to get COVID vaccines without being subject to status checks, the government said on Monday.
Anyone in Britain can register with doctors and access free frontline medical care, regardless of their immigration status.
Some data is shared with the interior ministry, which is responsible for border enforcement and investigating irregular immigration, including deporting those with no right to remain.
However, the health ministry has made clear to officials in the state-run National Health Service (NHS) that people receiving Covid-19 vaccinations, as well as testing and treatment, are not subject to any immigration status checks.
"Coronavirus vaccines will be offered to everyone living in the UK free of charge, regardless of immigration status," a government spokeswoman said.
"Those registered with a GP (general practitioner) are being contacted at the earliest opportunity and we are working closely with partners and external organisations to contact those who are not registered with a GP to ensure they are also offered the vaccine."
Britain is banking on its biggest ever vaccination programme as a way out of one of the world's worst outbreaks that has seen more than 112,000 deaths.
It has so far vaccinated more than 11 million people using either the Pfizer/BioNTech or Oxford/AstraZeneca shots.
The government has set itself a target of inoculating 15 million of the most vulnerable by the end of this week.
But ministers and health officials are concerned the rollout could be undermined by any segment of the population not getting the jabs.
It is not known how many people without a valid legal status are in Britain, with some estimates putting the figure as high as 1.3 million.
Meanwhile UK officials also fear large numbers within black African and south Asian communities could also be reluctant to take part in the vaccine drive, partly because of mistrust.
The British Medical Association (BMA), which has previously called for immigration data-sharing to be suspended during the pandemic, said the message that people's status will not be checked "must be communicated clearly and widely".
"For the vaccination programme to be successful it is crucial that we get as many people vaccinated as possible," John Chisholm, its ethics committee chair, told AFP in a statement.
"It is therefore vital that there are absolutely no barriers preventing migrant groups coming forward for vaccination."
Chisholm added it was particularly important "given the massively disproportionate impact of the pandemic on BAME (black, Asian and minority ethnic) populations".