he UK is facing a “significant reduction” in the amount of Covid-19 vaccines available from March 29 due to a cut in supply coming into the country.

“The Government’s Vaccines Task Force have now notified us there will be a significant reduction in weekly supply available from manufacturers beginning in week commencing 29 March, meaning volumes for first doses will be significantly constrained,” the letter circulating the NHS on Wednesday says.

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

The setback to the UK’s vaccination programme emerged as Health Secretary Matt Hancock led a coronavirus briefing at Downing Street on Wednesday.

Asked about the leaked letter by the BBC’s Laura Kuenssberg, Mr Hancock said vaccine supply is “always lumpy”, adding “we regularly send out technical letters to the NHS”.

The letter says local health leaders should “ensure no further appointments are uploaded to the National Booking System or Local Booking Systems from 1 to 30 April.”

It warns vaccine centres that inviting people outside of priority groups 1-9 is “only permissible in exceptional circumstances”, and orders them to prioritise all short-like stock of vaccine for use up to March 29.

The letter says that the reduction in supply “is likely to result in a reduction in workforce demand in hospital hubs and vaccination centres”, which means staff will need to be redeployed.

It finishes: “Our vaccination delivery programme was designed to be flexible, scaled up and diversified in line with fluctuating international vaccine supplies.

“Thank you for your continued efforts, and, as ever, we are hugely grateful for everything that you are doing to make the NHS’s part in the delivery of this programme the success that it is.”

The news will come as a blow to people in their 40s, who were expecting to be invited to receive their first dose within weeks.

Jonathan Ashworth, Labour’s Shadow Health Secretary, said: “People across the country will be anxious and worried that the booking of new first dose vaccination appointments will be paused from the end of March.

“Matt Hancock needs to explain exactly what these supply issues are and what he is doing to resolve them. Trying to dismiss or downplay the legitimate concerns of anxious people waiting for a vaccine is simply not good enough.”

During the press briefing, Mr Hancock insisted the government was still on course to vaccinated the top nine priority groups by mid-April.

Pressed again on the delay, the Health Secretary stuck to his line that supplies are “lumpy”.

“These supply schedules have moved up and down throughout,” he said, adding it was “par for the course”.

The Department of Health and Social Care (DHSC) said that more than 25 million people have received their first vaccine and 1.7 million have had their second vaccine.

Officials said the milestone brings people “one step closer to safely seeing our friends and family again”.

Mr Hancock said that the nation was “ahead of schedule” to offer a first dose to all over-50s by April 15.

In a statement on Wednesday evening, the DHSC insisted that the number of vaccinations varies due to supply but it remained “on track” to offer a first jab to over-50s by mid-April.

A spokesman said: “Health services across the UK are working tirelessly to vaccinate those most at risk and more than 25 million people have already received their first jab.

“The vaccination programme will continue in the coming weeks and more people will continue to receive first and second doses.

“As has been the case since the programme began, the number of vaccinations carried out over time will vary due to supply – but we remain on track to offer a first vaccine to over-50s by April 15 and all adults by July 31.”