Britain will have to slow its Covid-19 vaccine rollout next month due to a supply crunch caused by a delay in a shipment of millions of AstraZeneca shots from India and the need to test the stability of an additional 1.7 million doses.

Supply constraints are the biggest threat to Britain’s vaccine rollout – currently the swiftest among the world’s major economies – and health officials warned that the programme would face a significant reduction in supplies from March 29.

“It is true that in the short term we’re receiving fewer vaccines than we had planned for a week ago,” Prime Minister Boris Johnson, saying this was because of a delay in a shipment from India’s Serum Institute and because a batch in the UK needed to be retested.

“As a result, we will receive slightly fewer vaccines in April than in March, but that is still more than we received in February, and the supply we do have will still enable us to hit the targets we have set,” Boris Johnson said.

Earlier, health minister Matt Hancock had said that while Britain was currently in the middle of some “bumper weeks of supply”, a batch of 1.7 million vaccine doses had been delayed as it had to be retested for stability. He didn’t specify the manufacturer.

Britain is using vaccines made by Pfizer and AstraZeneca, with 10 million doses of the 100 million ordered from AstraZeneca coming from the Serum Institute.

A spokesman for the Serum Institute said it had delivered 5 million doses to Britain a few weeks ago, adding it would “try to supply later, based on the current situation and requirement for the government immunisation programme in India”.

Serum Institute Chief Executive Adar Poonawalla was quoted by the Daily Telegraph newspaper as saying that supplies were dependent on how many doses the Indian government allowed to go to the United Kingdom.

But, with Britain already at loggerheads with the European Union over vaccine exports, Johnson struck a conciliatory tone, saying he did not think India had blocked any deliveries and wanted to work with Europe too.

Pressed on whether the Indian government had stopped exports of vaccine to Britain, Johnson said: “No, no, there is a delay as there often is, caused for various technical reasons, but we hope to continue to work very closely with the Serum Institute, and indeed with partners around the world including on the European continent.”

Israel is the leader in vaccinating its population, followed by the United Arab Emirates, Chile and then the United Kingdom – and investors are watching closely to see which economies could recover first.

More than half of all adults in England have had their first COVID-19 vaccine. For the United Kingdom as a whole, just under half of adults have had their first dose.