The UK is to face a “significant reduction” in vaccine supplies from March 29, a letter from NHS England warns.

A letter sent to local vaccination sites across the country has warned volumes for first doses are going to “significantly constrained” at the end of this month.

Disruption to supplies is expected to last up to four weeks the government’s vaccine taskforce explaining that the limit to supplies is down to "reductions in national inbound vaccines supply", report Sky News.

The news could cause delays for younger age groups to receive the vaccine.

Vaccination centres are being told to close unfulfilled booking for the whole of April.

The most vulnerable will be prioritised with NHS England saying it is “vital” that second doses are delivered on schedule.

With the vaccine rollout moving at such a fast pace, the number of Brits due to receive their second dose is set to double in April.

NHS chief commercial officer Emily Lawson, who wrote the letter, added: "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."

Healthcare workers are now being advised that those under the age of 49 due to receive their vaccine should only be offered their first does in “exceptional circumstances”.

Health Secretary Matt Hancock argued the NHS letter warning vaccine supply will face a “significant reduction” was a “standard” technical letter.

He told the Downing Street press conference: “Vaccine supply is always lumpy and we regularly send out technical letters to the NHS to explain the ups and downs of the supply of the future weeks and what you are referring to is a standard one of those letters.”

The news comes as 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”.

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

Prime Minister Boris Johnson said: “This latest milestone is an incredible achievement – representing 25 million reasons to be confident for the future as we cautiously reopen society.

“Thank you once again to the brilliant NHS, scientists, armed forces, volunteers, and all those who’ve helped our rollout.”