Ministers say vitamin D might help protect against Covid-19, although the evidence to support this theory is not yet clear.
However, the vitamins will support bone and muscle health, which is particularly important this year when many people have been indoors for longer than usual due to lockdown.
Deliveries will start in January and those opting in will be sent four months’ supply, to last them through the winter months.
Ministers believe larger trials are needed to be sure of the link between vitamin D and fighting Covid-19.
But in the meantime, NICE and Public Health England have been asked to re-review the existing evidence, with their findings published next month.
The health secretary, Matt Hancock, said: “Because of the incredible sacrifices made by the British people to control the virus, many of us have spent more time indoors this year and could be deficient in vitamin D.
“The government is taking action to ensure vulnerable individuals can access a free a supply to last them through the darker winter months. This will support their general health, keep their bones and muscles healthy and crucially reduce the pressure on our NHS.”
Dr Alison Tedstone, chief nutritionist at Public Health England, said: “We advise that everyone, particularly the elderly, those who don’t get outside and those with dark skin, takes a vitamin D supplement containing 10 micrograms (400IU) every day.
“This year, the advice is more important than ever with more people spending more time inside, which is why the government will be helping the Clinically Extremely Vulnerable to get vitamin D.”