Matt Hancock has revealed a batch of 1.7m doses of the AstraZeneca Covid vaccine has been delayed because of the need to “retest its stability”.
The health secretary said a delay in a shipment of the vaccine from India had also impacted supply.
People in their 40s are likely to have to wait until May to get their jab, the government has announced.
Speaking in the Commons on Wednesday, Hancock said the government was still “on track” to offer the vaccine to everyone over 50 by April 15 and to all adults by the end of July.
But vaccination and rollout figures had previously suggested the NHS was set to easily beat that target.
Hancock said: “In April, supply is tighter than this month and we have a huge number of second doses to deliver. During April, around 12m people, including many colleagues in this House, will receive their second dose.
“These second doses cannot be delayed as they have to be delivered within 12 weeks of the first dose.
“In the last week, we’ve had a batch of 1.7m doses delayed because of the need to retest its stability.
“Events like this are to be expected in a manufacturing endeavour of this complexity and this shows the rigour of our safety checks.”
Hancock added the partnership with the Serum Institute of India was one the UK “can be proud of”, despite a delay.
“We have a delay in the scheduled arrival from the Serum Institute of India,” he told MPs.
“Now, I want to put on the record my gratitude to the Serum Institute of India for the incredible work that they’re doing producing vaccine not just for us in the UK, but for the whole world.
Hancock said despite the delay there will be “no weeks in April with no first doses”.
“There will be no cancelled appointments as a result of supply issues – second doses will go ahead as planned,” he said.
“Most importantly, the vaccine data published yesterday shows the life-saving impact of this vaccine. It’s not just that the vaccines are safe, it’s that they make you safe – you are much safer having had one.”