Around a million people will be sent a text message with booking instructions at 7am on Thursday.
The move, announced last week, followed the British medical regulator’s decision early last month to halt AstraZeneca doses for the under-30s, with alternatives such as the Pfizer vaccine now offered instead.
There have been 10.5 brain clots per million doses, according to the Medicines and Healthcare products Regulatory Agency (MHRA).
The country’s immunisation drive will soon be extended to younger age groups, with all adults expected to receive their first doses by late July, NHS sources told the PA news agency.
To date, more than 45 million shots have been administered in the UK, at least 18 million of which were second doses.
Dr Emily Lawson, of NHS England and Improvement, thanked NHS staff for hitting “yet another important milestone”, which she said was equivalent to almost 300,000 doses each day.
She added: “The success of the programme is not a happy accident – it's the result of people's determination to protect their patients, families and communities.”
“Now we need to keep up the momentum of the fastest and most successful NHS vaccination programme in history so when you get that text – book your appointment – and join the millions who have already been jabbed.”
Since they were eligible to book vaccination appointments two weeks ago, almost three-quarters of those aged 40 to 49 have received one dose, NHS England said.
Matt Hancock hailed the success of the “incredible” vaccination programme, which he said had already saved thousands of lives and helped the easing of lockdown restrictions.
The health secretary added: “Vaccines are our way out of the pandemic and I'm delighted we are now inviting people aged 38-39 to get their jabs.
”I urge everybody to get the vaccine as soon as they are eligible to protect yourself and your loved ones.“
Additional reporting by PA
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