An expected reduction in the UK's Covid vaccine supply in April is partly due to a delay in a delivery from India of five million Oxford-AstraZeneca doses.
The shipment, produced by the Serum Institute of India, has been held up by four weeks, reports the BBC.
UK Housing Secretary Robert Jenrick said neither a single factory nor a country was responsible for supply issues.
The Department of Health insists it is still on track to offer a first dose to all adults by the end of July.
NHS England warned of a reduction in supply in April in a letter to local health organisations on Wednesday.
A spokesperson for the Serum Institute said: "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."