People in England aged 70 and over, as well as those listed as clinically extremely vulnerable, will begin receiving offers of a coronavirus vaccine this week.
Prime Minister Boris Johnson said the move was a “significant milestone” in the nation’s vaccination programme.
It comes as 10 new mass vaccination hubs open across England.
Also from Monday, the UK is closing its travel corridors to protect against “as yet unidentified new strains” of Covid.
The expansion of the vaccination programme to the third and fourth priority groups comes after the number of people to receive a first dose rose to 3.8 million across the UK – more than have tested positive (3.4 million) since the pandemic began.
However, people in the top two groups – care home residents, those aged 80 and over and front-line healthcare workers – should still be prioritised for vaccinations, the Department for Health and Social Care said.
A further 298,087 people received their first dose of the vaccine on Saturday, and Health Secretary Matt Hancock said half of all those aged 80 and over had received at least one vaccine dose so far.