The shortage in the UK’s Covid-19 vaccine supply was caused by the delayed delivery of five million AstraZeneca-Oxford doses from India.

It was revealed on Wednesday that Britain faced a “significant reduction in the weekly supply” of vaccines next month, with the shipment from the sub-continent held up by four weeks.

Britain’s National Health Service was told not to make any further first dose appointments for younger people in April.

The Serum Institute, the world’s largest manufacturer of vaccines, said the doses that were due to arrive in the UK were needed for India’s domestic program, although the UK government refused to blame the company for the supply hiccup.

“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 immunization program in India,” a Serum Institute spokesman said.

The shortage comes as President of the European Commission Ursula von der Leyen did not rule out blocking exports of vaccines made on the continent to Britain. She also threatened to seize AstraZeneca factories and strip the pharmaceutical giant of its intellectual property rights.

The bump in UK supply will cause a bottleneck in the coming weeks, with a backlog of people requiring their second shot.

People will receive their booster injection as planned, the government said, and efforts will be made to inoculate those remaining from the top priority groups, including over-50s.

Younger people will have to wait a month longer than expected to receive their first shot but the government is confident it will meet its initial target of inoculating all adults by the end of July.

Asked whether doses earmarked for Britain were held back for India’s programme, UK Communities Secretary Robert Jenrick told the BBC on Thursday: “It would not be right for me to pin blame on any one manufacturer, factory or country – that is not the case.

“The manufacturers are working incredibly hard and it is to be expected we’d experience ups and downs along the road.”

Jenrick said the UK had less supply of the vaccine available for April but that people requiring their second dose “should have complete confidence” in their appointment going ahead.

“The month of April will be different – and it was always going to be – because I think this will be the month that second jabs exceed first jabs,” he said.

The supply shortage was revealed as Health Secretary Matt Hancock was delivering a press conference on Wednesday afternoon.

“The Government’s vaccines taskforce have now notified us there will be a significant reduction in weekly supply available from manufacturers beginning in week commencing March 29, meaning volumes for first doses will be significantly constrained,” a leaked letter to NHS operators said.

“They now currently predict this will continue for a four-week period, as a result of reductions in national inbound vaccines supply.”