A delay in the delivery of five million doses of the AstraZeneca coronavirus vaccine from India is thought to be behind a forthcoming reduction in the UK’s supply.

The delivery had been expected from the Serum Institute of India, the BBC reported, but has been held up by four weeks.

A letter to health leaders in England, published on Wednesday, warned of a “significant reduction in weekly supply” of the vaccine from March 29, “meaning volumes for first doses will be significantly constrained”.

The letter from Emily Lawson, NHS chief commercial officer, and Dr Nikita Kanani, medical director for primary care, said people “aged 49 years or younger should not be offered vaccination” unless they are in a higher priority group, such as being clinically vulnerable.

It added that the UK’s vaccines taskforce predicts the shortfall will last four weeks “as a result of reductions in national inbound vaccines supply”.

The Department of Health has said the Government is still on track to offer a first dose to all adults by the end of July.

A spokesperson for the Serum Institute of India told the BBC: “Five million doses had been delivered a few weeks ago to the UK and we will try to supply more later, based on the current situation and the requirement for the government immunisation programme in India.”

AstraZeneca has partnered with the institute, which is the world’s largest vaccine manufacturer, for supplies to the Indian government but also to other countries, including low and middle-income ones.

Cabinet minister Robert Jenrick conceded the rollout of vaccines would be slightly slower than expected because of the shortage.

The Housing Secretary told BBC Breakfast: “We are experiencing some supply issues so it does mean the vaccine rollout will be slightly slower than we may have hoped but not slower than the target we set ourselves.

“We’re going to move forward as quickly as we possibly can but it won’t be as fast as we might have hoped for a few weeks, but then we have every reason to believe that supply will increase in the months of May, June and July.”

The European Medicines Agency (EMA) will publish its findings on the AstraZeneca vaccine on Thursday, after more than a dozen European countries suspended rollout of the jab over a small number of cases of blood clotting.