The number of people getting the first dose of their Covid-19 jab will be “constrained” as a result of a reduction in the supply available, health officials have said.

NHS leaders said there will be a “significant reduction” in the weekly vaccine supply available from the week beginning March 29.

A letter to regional NHS bosses says that the reduction will continue for a “four-week period”.

Local health leaders have been told to focus efforts on the top priority groups in the letter, signed by Dr Nikita Kanani, medical director for primary care for the NHS in England, and Emily Lawson, chief commercial officer.

“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 29 March, meaning volumes for first doses will be significantly constrained,” the letter states.

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

The letter adds that inviting people for jabs who are not in the top nine priority groups is “only permissible in exceptional circumstances”.

It adds: “Those aged 49 years or younger should not be offered vaccination unless they are eligible via a higher cohort because they are e.g., clinically vulnerable, unpaid carer or frontline health and care workers.”

Last week Vaccines Minister Nadhim Zahawi told the Women and Equalities House of Commons Committee that the NHS could expect a “big uplift in supply” in the second half of March.

It comes as the Government celebrated reaching the milestone of getting 25 million adults jabbed in the first 100 days of the vaccination programme.

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.

Ministers have pledged that all adults will be offered a vaccine by the end of July.

Officials said that 95% of people aged 65 and over have had their first dose.

And nine in 10 of those clinically extremely vulnerable have received a first jab.

Overall, health services across the UK vaccinated 25,273,226 people between December 8 and March 16 with first doses of the Pfizer/BioNTech and Oxford/AstraZeneca vaccines.

Half the adult population of the UK is 26.3 million.

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.”

Health Secretary Matt Hancock added: “This is an extraordinary feat, coming exactly 100 days after Margaret Keenan received the first authorised jab in the whole world.

“It has been a national mission, one of the biggest logistical exercises since the war and I’d like to thank everyone who played their part, including every NHS vaccinator, GPs, pharmacists, volunteers and the armed forces for their crucial role in every corner of the UK.

“We’re ahead of schedule to offer a first dose to all in these groups by the 15 April and I urge everybody eligible to come forward.”

Mr Zahawi added: “This is an incredible milestone and moves us one step closer to safely seeing our friends and family again.

“The vaccination programme will continue to expand over the coming weeks and more people will receive their second doses.”

The NHS in England has invited all over-50s to receive their jab.

It said that invitations are being sent out nationally to people in this priority group.

The health service also encouraged people who have already been invited for a jab to take up the offer.

Sir Simon Stevens, head of the health service in England, said: “Just 100 days since the NHS gave the world’s first Covid jab outside of clinical trials, our vaccine programme passes another milestone as we now invite everyone aged 50 and over to book their vaccination.”