There is expected to be a significant reduction in weekly supply of the Covid-19 vaccines from the end of March.
And health leaders have said volumes for first doses will be significantly reduced.
But what does this mean for the vaccination programme?
Here some of the key questions are answered:
What is behind the delay?
A delivery of five million doses of the AstraZeneca vaccine from the Serum Institute of India has been held up by four weeks.
This is partly behind some of the reduction in the UK’s supply.
But indications from the Government are that the delay is not down to one nation, and that a number of global manufacturers are experiencing issues.
– What does this mean for the vaccine rollout?
Health leaders in England have said that from March 29 volumes for first doses will be significantly constrained.
The shortfall is predicted to last four weeks.
Professor Martin Marshall, chairman of the Royal College of GPs said the focus in April will be on giving second doses to people who were vaccinated earlier in the year.
The Joint Committee on Vaccination and Immunisation has said there should be between four and 12 weeks between doses.
But what about the vaccination targets?
The Department of Health and Social Care says the Government is still on track to offer a first dose to all adults by the end of July.
Health Secretary Matt Hancock has said the April 15 target for vaccinating over-50s will also still be met.
So who does the delay affect?
People in their 40s are likely to have to wait until May to get their Covid-19 vaccine.
It had previously been hoped that vaccination of this group would start in April, after all of those over-50 had received their first jab.
– What about the Moderna vaccine?
The jab from US biotech firm Moderna is not expected to arrive until the spring, although a specific date has not yet been given.
After the vaccine was approved by regulators in January, the Government purchased an additional 10 million doses of the vaccine on top of its previous order of seven million.