Video report by ITV News Health Editor Emily Morgan
The UK is to experience a "significant reduction" in vaccine supply from the end of this month, the NHS has said.
The "supply constraint" will last for a month from the week beginning March 29, and local vaccination centres and services have been told by NHS England to close unfilled bookings from March 29.
Vaccine centres and services have also been advised to "ensure no further appointments are uploaded to the National Booking System or Local Booking Systems from April 1 to 30".
It means millions of people under 50 face the prospect of having to wait until after April 30 for their first vaccine appointment.
A letter from NHS England leaders to local health organisations said: "The government's Vaccines Taskforce have now notified us that there will be a significant reduction in weekly supply available from manufacturers beginning in the week commencing March 29, meaning volumes for first doses will be significantly constrained."
Speaking about the letter, Health Secretary Matt Hancock said "vaccine supply is always lumpy" and the government regularly sends "technical letters to the NHS to explain the ups and downs of vaccine supply over the future weeks".
The advice to vaccine centres means they must focus on delivering second doses from March 29 to April 30, rather than moving on to give more people their first dose.
The letter said vaccines should not be offered to anyone outside the top nine priority groups during that time period, unless under "exceptional circumstances".
A number of areas, including Nottingham, had this week began offering vaccines to under 50s, however that will have to pause when the supply contracts.
He added: "I can assure... there is enough supply and we have the NHS across the UK ready to be able to deliver on the offer of a vaccine to all over 50s by April 15."
Anyone over 50 is being advised to book themselves a vaccination via the NHS online.
ITV News understands that the shortages may be due to the delayed delivery of 10 million doses of the AstraZeneca vaccine from India which was due this weekend.
However, AstraZeneca said there are no issues with the domestic supply chains and that it is confident that its commitments are on track.
A spokesperson said: “Our UK domestic supply chain is not experiencing any disruption and there is no impact on our delivery schedule."
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A Pfizer spokeswoman said deliveries “remain on track” for the first quarter of its 40 million dose agreement with the UK.
She added: “We will work closely with the government to ensure this remains the case; our overall projected supply for Q2 remains unchanged and we are on course to continue to deliver a steady supply of vaccines to the UK, April through to June, in line with our contractual commitments.
“Based on current projections, we believe that we can deliver more than two billion doses of the Pfizer/BioNTech vaccine worldwide by the end of 2021 – an increase from the 1.3 billion doses initially projected. As of March 11, we have shipped 160 million doses of our vaccine worldwide and we are tripling the number of doses we deliver to the EU in Q2 compared to Q1.”
The news of vaccine supply issues comes as the EU threatens to block the export of jabs to countries with high inoculation rates, such as the UK.
European Commission president Ursula von der Leyen told reporters in Brussels the EU was seeking to block exports of coronavirus vaccines to countries with higher vaccination rates.
Earlier the health secretary said the UK was "ahead of schedule" in its target to vaccinate all those in the top nine priority groups by April 15.
Watch Wednesday's government coronavirus news conference with Matt Hancock
Official figures show 25,273,226 people have had their first doses of either the Pfizer or Oxford/AstraZeneca vaccines.
At least 1,759,445 of those people have had their second dose.
Earlier, 56-year-old Prime Minister Johnson revealed the NHS had been in touch to book his first vaccine appointment, which he said he would be receiving "very shortly".
DHSC said it expected the UK have vaccinated more than half of the entire adult population by the end of the week.
A DHSC spokesperson said: “Health services across the UK are working tirelessly to vaccinate those most at risk and more than 25 million people have already received their first jab.
“The vaccination programme will continue in the coming weeks and more people will continue to receive first and second doses.
“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.”
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