Supply of the Covid-19 vaccine is to be hit by a “significant reduction” from the end of March, raising questions over whether under-50s will face a delay in getting the jab and the UK government’s vaccination targets.

A letter from the NHS in England to local health leaders published on Wednesday states that “volumes for first doses will be significantly constrained” for four weeks.

At a Downing Street press conference after the letter emerged, health secretary Matt Hancock repeatedly dodged questions about the reasons for the delay, saying only that “vaccine supply is always lumpy and we regularly send out technical letters to the NHS”.

But he insisted the UK is still “on course” to ensure everyone aged 50 and over will be able to get vaccinated by April 15.

Asked about the letter, Hancock said: “Supply is always lumpy and we are on course to deliver the offer that everybody who is aged 50 and above will be able to get vaccinated by the 15th of April. I recommit to that today.

“And, of course, these supply schedules have moved up and down throughout this whole rollout. It’s absolutely par for the course and that’s a normal operation letter.

“We are committed to all adults being able to get the jab by the end of July and we are on track to deliver on that commitment.”

The letter from NHS England leaders states: “The government’s Vaccines Task Force have now notified us that there will be a significant reduction in weekly supply available from manufacturers beginning in the week commencing March 29, meaning volumes for first doses will be significantly constrained.

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

Fears over supply come as the European Union threatened on Wednesday to ban exports of Covid-19 vaccines to Britain to safeguard scarce doses for its own citizens.

Hancock said at the briefing there would be a focus on vaccinating the most vulnerable before moving on to the over-40s but, under pressure, conceded that the target of vaccinating all adults by the end of July remained.

He told the Downing Street press conference: “We’re on track to offer a first dose to everyone in priority groups 1 to 9 by April 15. While we deliver on that commitment, we also want to ensure that this offer reaches everyone in groups 1 to 9.

“At the same time as opening up offers of vaccinations to all those who are 50 or above, we are going to do whatever it takes to reach all those in the most vulnerable groups who haven’t come forward yet before we move onto the next cohort, which is people in their 40s.

“Before we forge ahead I want us to be confident that we’ve done everything we can to protect those most in need of protection and we will do all we can and do everything necessary to deliver the supplies that are contractually committed to protecting people in this country.”

Against a backdrop of a potential third wave of the pandemic on the continent, European Commission head Ursula von der Leyen said the bloc was facing “the crisis of the century”. 

Von der Leyen said the flow of vaccine products was smooth with the US but aired frustration over a lack of deliveries from AstraZeneca in the UK. She said 10 million doses had gone from EU plants to the former member state.

“We are still waiting for doses to come from the UK,” von der Leyen said in the latest sign of souring ties between Britain and the 27-nation bloc since Brexit.

“If this situation does not change, we will have to reflect on how to make exports to vaccine-producing countries dependent on their level of openness. We will reflect on whether exports to countries with higher vaccination rates than us are still proportionate.”

British foreign minister Dominic Raab hit back, accusing the Commission of brinkmanship and calling for an explanation after the government had received assurances that the EU did not intend to curb exports which are under contract.