The government set Monday as the deadline to deliver a first vaccine dose to everyone in its top four priority segments, including all those aged 70 and over, a goal it looks likely to reach.
According to the latest official figures, 14.56 million people have received a shot, allowing health officials to expand the programme to those aged 65 to 69, and to other clinically vulnerable people.
The National Health Service (NHS) said about 1 million people had already received invitations.
"Every jab in the arm is another step closer to returning to normality in the future and that's why we're now inviting the next priority group to book their appointments," Vaccine Minister Nadhim Zahawi said in a statement.
Britain, which has recorded more than 120,000 deaths from COVID-19, was the first Western country to begin mass vaccinations in December, and is ahead of other European countries in rolling out the shots.