All over-50s can now book their coronavirus jab via the NHS website as the rollout begins moving on to to younger age groups.
The NHS has update its website to allow all over-50s to book online - up until today it had being allowing over-55s only to schedule their own appointments.
The UK had given first doses of the vaccine to nearly 25 million people by Tuesday.
Reports in recent weeks speculated people aged in their 40s could be offered the jab by the end of this month at the current pace of the UK's vaccine programme.
The speed is expected to ramp up from today in a rapid expansion drive as vaccine dose stocks increase.
Boris Johnson has set the target of vaccinating all UK adults by the end of July, but it is hoped the target could be met even sooner in one step towards securing Britain's route out of lockdown.
It comes as health authorities encourage confidence in the Oxford AstraZeneca vaccine.
See full list of who is eligible to book a vaccine online with the NHS below...
A number of European Union countries had halted the use of the jab amid concerns it was linked to blood clots.
AstraZeneca has found no evidence of increased blood clot risk from the vaccine, and the UK Government has said it has full confidence it is safe.
It analysed its safety review of 17 million people who had received the jab and found no increased risk, after some countries suspended the use of its jab.
The World Health Organisation has also said the rollout of the AstraZeneca vaccine should continue, as the issue divides Europe.
Professor Jeremy Brown from the UK's Joint Committee and Vaccination and Immunisation (JCVI), which organises the priority list of the rollout, told Good Morning Britain this morning the blood clots reported in Europe were "very, very rare events which may be occurring in three or four people per million people vaccinated.
"And yet, on the other side, that is against the fact that if you vaccinate a million people with the AstraZeneca vaccine, you're going to save 1,000 lives.
"They have taken the precautionary principle about doing no harm and they have used it in the wrong way - by looking at not using the vaccine whereas in fact you should continue using the vaccine unless there's incredibly good reasons not."
Prof Brown added he did not believe the "specific unusual clot in the vein of the brain" that is at the root of concerns in Germany "will turn out to be linked to the vaccine anyway - this is an incredibly rare event".
He said: "We don't know how often it occurs, even if people have not been vaccinated. It's a complication of actually having acute Covid infection itself, and therefore it's very, very rare and unlikely to be linked to the vaccine.
"Using that as a reason to stop using the vaccine when we know the vaccine prevents 85 to 90% admission to hospital is not sensible."
The UK has shot ahead of EU countries in its drive to vaccinate the population.
Official antibodies figures this week revealed three-quarters of all over-80 in England now have immunity against Covid-19.
The promising new figures from the Office for National Statistics (ONS) showed that by March 3 the antibody levels had increased significantly to 75.7% of over-80s - compared with 56.4 per cent in the previous fortnight.
Full list of who is eligible to book a jab online
The NHS is currently offering the vaccine to people most at risk from coronavirus, including frontline health and care workers.
The following groups can book online:
- people aged 50 and over
- people at high risk from coronavirus (clinically extremely vulnerable)
- people who live or work in care homes
- health and social care workers
- people with a condition that puts them at higher risk (clinically vulnerable)
- people with a learning disability
- people who are a main carer for someone at high risk from coronavirus
You can book your vaccine online on the NHS website here.
