Boris Johnson has insisted the vaccination programme and route out of lockdown remain on track despite a shortfall in the expected supply of jabs.
A delay in deliveries from India and the need to retest a batch of 1.7 million doses is behind the issues with vaccine supply in April.
The problem with a shipment from the Serum Institute of India (SII) has been blamed by the body’s chief on the country’s government, although Mr Johnson said Narendra Modi’s administration has not stopped any exports.
Mr Johnson said: “We have always said that in a vaccination programme of this pace and this scale, some interruptions in supply are inevitable.
“It is true that in the short term we are receiving fewer vaccines than we had planned for a week ago, that is because of a delay in a shipment from the Serum Institute – who are doing a herculean job in producing vaccines in such large quantities – and because of a batch that we currently have in the UK that needs to be retested as part of our rigorous safety programme.
“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.”
The over-50s and the clinically vulnerable will still be offered a first dose by April 15, and second doses will be available to around 12 million people in April.
Every adult will be offered a first dose by the end of July, as planned, he said.
“Our progress along the road to freedom continues unchecked, we remain on track to reclaim the things we love, to see our families and friends again, to return to our local pubs, our gyms and sports facilities and, of course, our shops,” he said.
SII chief executive Adar Poonawalla told The Telegraph the delay to a shipment of millions of doses was “solely dependent on India and it has nothing to do with the SII”.
“It is to do with the Indian government allowing more doses to the UK,” he said.
But Mr Johnson told a Downing Street press conference “the Indian government has not stopped any export” but “there is a delay… as there is very frequently in vaccine rollout programmes”.
Another potential supply difficulty could be European Commission president Ursula von der Leyen’s threat to block exports of jabs from the EU to countries with higher vaccination rates that do not offer reciprocal supplies of vaccine.
Mr Johnson said: “It’s very important to stress whatever you may hear about the pressures that different countries are under to deliver vaccines for their public, these vaccines are a multinational effort, they are produced as the result of international co-operation and I want to stress that we in the UK will continue to view it in that spirit.
“We don’t have any bans on exporting stuff and we will continue to co-operate with our European friends.”
The issues with supply came as regulators in the UK and Europe reaffirmed the safety of the Oxford/AstraZeneca jab – a vaccine Mr Johnson will receive on Friday.
The Prime Minister said: “The Oxford jab is safe and the Pfizer jab is safe. The thing that isn’t safe is catching Covid, which is why it is so important that we all get our jabs as soon as our turn comes.”
The latest figures showed:
– Up to March 17, 25,735,472 first doses of vaccine have been administered in the UK, a rise of 462,246 on the previous day.
– A further 91 people had died within 28 days of testing positive for Covid-19 as of Thursday, bringing the UK total to 125,926 by that measure.
– As of 9am on Thursday, there had been a further 6,303 lab-confirmed cases in the UK.
The use of the AstraZeneca vaccine had been suspended in countries across Europe over concerns about blood clots in people who have received the jab.
But the European Medicines Agency confirmed on Thursday the vaccine is “safe and effective” and its benefits outweigh any risks.
However, the regulator said it “cannot rule out definitively” a link between “a small number of cases of rare and unusual but very serious blood clotting disorders” and the vaccine, though investigations were ongoing.
In the UK, the Medicines and Healthcare products Regulatory Agency (MHRA) also concluded that any link between the jab and clots is unproven, and the benefits of the vaccine outweigh any risks.
Five men in the UK have suffered an “extremely rare” blood clot problem in the brain after having the AstraZeneca vaccine, with one of them subsequently dying, though no causal link with the jab has been established.
The MHRA said it was looking at the reports but stressed the events were “extremely rare” and there was a possibility they could have been caused by Covid itself.
Professor Chris Whitty, England’s chief medical officer, said the controversy over the AstraZeneca vaccine had not affected the take-up rate.
“The general public is, as always, sensible and steady on this,” he said. “They understand this is a dangerous disease.”
The looming problems with vaccine supply emerged on Wednesday when NHS England told health leaders to expect a significant shortfall in vaccine doses from March 29 for about four weeks.
It said people under 50 should not be booked in for first appointments unless they fell into a higher priority group, such as being clinically vulnerable.
The move means the under-50s could now have to wait until May to get a vaccination, despite doctors having planned to start on that group in April.
Scotland’s First Minister Nicola Sturgeon said Scotland will have 500,000 fewer coronavirus vaccine doses over the next month than anticipated.
However, Northern Ireland has suggested it may still be able to start vaccinating people in their 40s from mid-April.