About half a million more people in England are being invited to book their Covid-19 jab from Monday, as the vaccine rollout opens to 44-year-olds.
Two-thirds of the previous age group - 45 to 49-year-olds - have received their first dose.
The NHS said it would set out when 40 to 43-year-olds would be able to book appointments "in the coming days", as supply allows.
It comes as a TV advert is launched to encourage under-50s to get vaccinated.
The total number of vaccine doses given in the UK reached almost 46.3 million on Sunday, with a further 498,430 second doses and 142,215 first doses reported.
More than 33.6 million people have now received their first dose and almost 12.6 million are fully vaccinated.
Health Secretary Matt Hancock said: "Vaccines are helping us get back to doing the things we have missed."
Health Secretary Matt Hancock said: "Vaccines are helping us get back to doing the things we have missed."
Northern Ireland is opening its vaccination programme to people aged 35 to 39, while Wales has been asking over-40s to book appointments and Scotland has been inviting over-45s for vaccination.
Meanwhile, Scotland is taking its biggest step out of lockdown since December as shops reopen along with cafes, pubs and restaurants - with some restrictions. Outdoor hospitality is also opening up in Wales.
The TV advert promoting the Covid-19 vaccine - featuring the slogan "every vaccination gives us hope" - is predominantly aimed at people aged under 50 who are being invited for their first dose, the Department of Health and Social Care said.
Showing scenes from vaccination centres across the country, it also aims to encourage over-50s to ensure they receive their second jab for the best possible protection.
It will be accompanied by a poster campaign highlighting how the Covid-19 jab helps to reduce infections, hospital admissions and deaths.