The UK is going to face a "significant reduction" in coronavirus vaccine supplies from 29 March, for an estimated month-long 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.
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".
Sky's health correspondent, Ashish Joshi, has been told that the shortage has nothing to do with ongoing supply tensions with the EU - instead, it is down to a "global supply issue" with AstraZeneca.
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.
This could result in disappointment for those who were hoping to get their vaccine soon, and could cause delays for younger age groups.
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.
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."
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.
© Sky News 2021
