The UK is going to face a "significant reduction" in coronavirus vaccine supplies from 29 March, for an estimated four-week period, according to a letter from NHS England.
The letter - which has been sent to local vaccination sites across the country - says volumes for first doses are going to be "significantly constrained".
Healthcare providers have now been told that those aged 49 and under should only be offered a COVID-19 vaccine in "exceptional circumstances" - such as if they are clinically vulnerable or a frontline care worker.
Medics will concentrate on those in cohorts 1 to 9 - everyone over the age of 50 and those vulnerable to COVID.
Disruption is predicted to last for four weeks, and the government's vaccines taskforce has told the health service that this is linked to "reductions in national inbound vaccines supply".
It had been thought the problem was down to a global supply issue with AstraZeneca but in a statement the company said: "Our UK domestic supply chain is not experiencing any disruption and there is no impact on our delivery schedule."
The other approved vaccine maker supplying the UK, Pfizer, said: "We have an agreement with the government to supply 40 million doses of the Pfizer/BioNTech vaccine to the UK by the end of the year.
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"While the details of this agreement are confidential, we can confirm that Q1 deliveries remain on track and continue to progress in line with the monthly schedule, agreed in advance with the vaccines taskforce."
From today, vaccination centres are being told to close unfilled bookings from 29 March onwards - and to ensure no further appointments are scheduled for the whole of April.
Despite the strongly worded language in the letter, Health Secretary Matt Hancock told the Downing Street briefing: "Vaccine supply is always lumpy and we regularly send out technical letters to the NHS to explain the ups and downs of the supply over the future weeks. What you're referring to is a standard one of those letters."
Mr Hancock also stressed that the UK remains on track to offer a first dose to all 32 million people in the top nine priority groups by 15 April.
He said: "We are going to do whatever it takes to reach all those in the most vulnerable groups who haven't come forward yet before we move on to the next cohort, which is people in their 40s," he added.
When asked whether the letter means under-50s will have to wait an extra month to get their jab, he reaffirmed a commitment that all adults will be able to receive a vaccine by the end of July.
In the letter, NHS England said that it is "vital" that second doses of the COVID-19 jab are delivered on schedule, with the number of Britons due to receive their final vaccine set to double from the beginning of April.
"We must take this time to deliver protection to the most vulnerable," it added.
Efforts are also being stepped up to minimise wastage with existing supplies.
NHS chief commercial officer Emily Lawson, who wrote the letter, added: "Our vaccination delivery programme was designed to be flexible, scaled up and diversified in line with fluctuating international vaccine supplies.
"Thank you for your continued efforts, and, as ever, we are hugely grateful for everything that you are doing to make the NHS's part in the delivery of this programme the success that it is."
A Department of Health and Social Care spokesperson said: "As has been the case since the programme began, the number of vaccinations carried out over time will vary due to supply - but we remain on track to offer a first vaccine to over 50s by 15 April and all adults by 31 July."
The news came hours after the UK announced that more than 25 million people have now had their first dose of a coronavirus vaccine - with everyone over the age of 50 invited to come forward to get a jab.
Tensions have also been rising with the EU, with European Commission President Ursula von der Leyen threatening to block vaccine exports to the UK.
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