The NHS has invited people over the age of 45 to book their Covid-19 vaccine appointment.
It signals the start of “Phase 2” of the vaccination programme – which involves offering vaccines to healthy adults aged under 50.
The move comes as it was announced that the Government has met its target of offering the vaccine to all adults over the age of 50.
In England it has been estimated that 19 in 20 of those who have already been offered the jab have taken up the offer.
One vaccination expert said that it was “vitally important” to vaccinate “the last few per cent”, or they could get infected and end up in hospital.
Professor Jeremy Brown, from University College London Hospitals and member of the Joint Committee of Vaccination and Immunisation (JCVI), told the Today programme: “The problem here is that 5-10% have not been vaccinated.
“When the virus re-circulates through the community they could get infected and end up in hospital.
“So it’s vitally important that we get that last few per cent.”
Until now, the NHS had been focusing on offering vaccines to those deemed to be at highest risk – people over the age of 50, those deemed to be “clinically extremely vulnerable” and health and social care workers.
When its online booking system opened up to include people over the age of 45, the website crashed for many.
The announcement comes despite an earlier warning by the NHS in England of a “significant reduction in weekly supply” during April, meaning volumes for first doses would be “significantly constrained”.
The Prime Minister hailed the “hugely significant milestone” in the vaccination programme of reaching the April 15 target.
In a statement, Boris Johnson said that the Government remained on course to offer a first jab to all adults in the UK by the end of July.
“We have now passed another hugely significant milestone in our vaccine programme by offering jabs to everyone in the nine highest risk groups,” he said.
“That means more than 32 million people have been given the precious protection vaccines provide against Covid 19.
“I want to thank everyone involved in the vaccine rollout which has already saved many thousands of lives.
“We will now move forward with completing essential second doses and making progress towards our target of offering all adults a vaccine by the end of July.”
The news comes as the Moderna vaccine was used for the first time in England.
The Government has ordered 17 million doses of the jab – enough for 8.5 million people.
Meanwhile, Ireland became the latest country to restrict the use of the AstraZeneca vaccine, saying it should not be given to people under the age of 60, amid concerns over possible links to rare blood clotting events.
In the UK, the advice is that it should not be administered to those aged under 30.
In all, the Government said almost 40 million doses of the Pfizer and AstraZeneca vaccines have been delivered since the rollout began in the UK in December, including 32 million first jabs and more than seven million second doses.
NHS England chief executive Sir Simon Stevens said 19 out of 20 over-50s had now received a first jab.
“Thanks to our NHS nurses, doctors, pharmacists, operational managers and thousands of other staff and volunteers, the NHS Covid vaccination programme is without a doubt the most successful in our history,” he said.
“It’s one of our tickets out of this pandemic and offers real hope for the future.”
Throughout the current month, the health service has prioritised second doses of the Pfizer and AstraZeneca vaccines with a record 475,230 people receiving their second jab on Saturday.
Meanwhile, there will be a surge in testing in the boroughs of Lambeth and Wandsworth in south London following the identification of a number of cases of a variant of the virus first found in South Africa.
Dozens of people were told to isolate after the Department for Health and Social Care said there had been 44 confirmed cases of the strain, with a further 30 suspected.
It came as scenes posted on social media showed revellers gathering in Soho, central London, following the latest easing of lockdown restrictions in England, prompting fears that social distancing guidelines were being ignored.