NHS leaders said there will be a “significant reduction” in the vaccine supply from the week beginning March 29.
A letter to regional NHS bosses says that the reduction will continue for a “four-week period”.
As a result, health leaders have said that people under the age of 50 should only get the jab if they are in a priority group for the vaccine.
Health Secretary Matt Hancock told the Downing Street press conference that the nation was “on course” to meet the target of offering a first dose to all over-50s by April 15.
Local health leaders have been told to focus efforts on the top priority groups in the NHS letter, signed by Dr Nikita Kanani, medical director for primary care for the NHS in England, and Emily Lawson, chief commercial officer.
They may also need to redeploy staff working in vaccination centres to other settings to try and increase uptake among the priority groups, it adds.
“The Government’s Vaccines Task Force have now notified us that there will be a significant reduction in weekly supply available from manufacturers beginning in the week commencing 29 March, meaning volumes for first doses will be significantly constrained,” the letter states.
“They now currently predict this will continue for a four-week period, as a result of reductions in national inbound vaccines supply.”
The letter adds that inviting people for jabs who are not in the top nine priority groups is “only permissible in exceptional circumstances”.
It adds: “Those aged 49 years or younger should not be offered vaccination unless they are eligible via a higher cohort because they are e.g., clinically vulnerable, unpaid carer or frontline health and care workers.”
Ministers have pledged that all adults will be offered a vaccine by the end of July.
Asked about the issue at the Downing Street coronavirus briefing, Mr Hancock said: “Supply is always lumpy and we are on course to deliver the offer that everybody who is aged 50 and above will be able to get vaccinated by the 15th of April. I recommit to that today.
“We are committed to all adults being able to get the jab by the end of July and we are on track to deliver on that commitment.”
He added: “We regularly send out technical letters to the NHS to explain the ups and downs of the supply of the future weeks and what you are referring to is a standard one of those letters.”
He said there would be a focus on vaccinating the most vulnerable before moving on to the over-40s
“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.
“Before we forge ahead I want us to be confident that we’ve done everything we can to protect those most in need of protection and we will do all we can and do everything necessary to deliver the supplies that are contractually committed to protecting people in this country.”
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.”
AstraZeneca has previously said that it is confident that its commitments are on track.
It comes as the Department of Health and Social Care (DHSC) said that more than 25 million people have received their first vaccine and 1.7 million have had their second vaccine.
Officials said that 95% of people aged 65 and over have had their first dose, and nine in 10 of those clinically extremely vulnerable have received a first jab.
The figures, which come on the 100th day of the vaccination programme, show that overall, health services across the UK vaccinated 25,273,226 people between December 8 and March 16 with first doses of the Pfizer/BioNTech and Oxford/AstraZeneca vaccines.