The UK's National Health Service said there would be "a significant reduction" in COVID-19 vaccine supplies from March 29.
"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 NHS said in the letter.
"They now currently predict this will continue for a four-week period, as a result of reductions in national inbound vaccines supply."
The letter said that vaccination centres and pharmacies should close unfilled bookings from March 29.
UK health minister Matt Hancock said however that the country was on track to meet its target to vaccinate everyone over the age of 50 by mid-April, calling the NHS letter "normal" and "par for the course".
"These supply schedules have moved up and down throughout the rollout," Hancock said, adding that the UK was "committed to all adults being able to get the jab by the end of July".
The country has already been highly praised internationally for its vaccine rollout - having given a first dose to 25 million people.
Vaccinations reduce hospitalisations and deaths
UK officials said at a press conference that there was evidence from those vaccinated in the country that a single dose of a COVID-19 vaccine reduces the chance of getting the disease by 60% and reduces the chance of hospitalisation by 80%.
If you live with someone who is vaccinated, you have a 30% lower risk of catching COVID-19, Hancock said.
This is translating into a reduction in infections and deaths, said Mary Ramsay, head of immunisation at Public Health England.
"Every day we vaccinate more people we are preventing more deaths," she said.
Fears about AstraZeneca's vaccine and links to blood clots
UK officials said that the country's regulator continues to view the benefits of the AstraZeneca coronavirus vaccine as outweighing the risks.
"There’s a lot of evidence emerging now that is reassuring that there is no overall excess signal or increased risk," said Jonathan Van-Tam, England's deputy chief medical officer.
He emphasised that while there is "no evidence of increased risk", there is "a lot of evidence that the vaccine is actually saving lives."
Several European countries including France and Germany have suspended AstraZeneca vaccinations over concerns that it could cause blood clots in some people, despite some experts insisting that the link is unproven.
"All medicines have side effects and all medicines have benefits and that’s the whole point that you have to look at both sides and say how big are the benefits in relation to the risks," Van-Tam added, comparing it to paracetamol which many people have at home.
He said that rare side effects include allergic reaction, difficulty breathing, fever, loss of appetite and jaundice.
"Those are documented rare side effects of paracetamol but we all understand the benefits of them and this is no different a situation," he said, re-emphasising that the link to blood clots remains unproven.