Children aged six months to four years, who are deemed to be in high-risk groups, will be offered a Covid vaccination for the first time.
This includes infants with medical conditions, such as cystic fibrosis, heart problems and compromised immune systems.
The jabs will start being given from mid-June in England. The other nations are yet to announce rollout dates.
It follows a recommendation by the government's vaccine advisers.
The Joint Committee on Vaccination and Immunisation (JCVI) said while young children were at low risk of severe illness, those with medical conditions were seven times more likely to require hospital treatment.
The JCVI said it has been monitoring data from the US, which has been vaccinating children in these groups since June 2022.
It said the data showed the most common side effects were irritability, crying, drowsiness and fever.
Eligible children will be offered two doses of the Pfizer vaccine, with an interval of eight to 12 weeks between the first and second doses.
Dr Mary Ramsay, head of immunisation at the UK Health Security Agency (UKHSA), said: "The extra protection offered by the vaccine could be important for young children in clinical risk groups who are at greater risk of severe illness.
"The virus is not going away, so I would encourage all parents to bring their child forward if they are eligible.
"Parents should wait to be contacted by their local health professionals," she added.
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