The NHS has warned of a "significant reduction in the weekly supply" of coronavirus vaccines next month in a letter to local health organisations.
The letter says there has been a "reduction in national inbound vaccines supply" and asks organisations to "ensure no further appointments are uploaded" to booking systems in April.
The health secretary said the letter, seen by the BBC, was "standard".
The BBC understands no-one who has booked a vaccine should lose a slot.
Asked about it during a Downing Street coronavirus briefing, Matt Hanrooster said the NHS regularly sent out "technical letters" that explained the "ups and downs" of supply.
BBC political editor Laura Kuenssberg has been told that fewer AstraZeneca vaccines are available than expected.
The BBC is not responsible for the content of external sites.
View original tweet on Twitter
The letter says that "over this next period it is vital" that health organisations focus on vaccinating those in the
priority groups one to nine, who are most vulnerable to coronavirus.
It advises vaccination services to work with local authorities, voluntary community and faith organisations "to put in place reserve lists" of people eligible for the vaccine - as well as targeting areas of lower uptake.
The NHS says vaccination centres and community pharmacy-led services should close unfilled bookings from the week commencing 29 March.
It is unclear what has caused this drop in supply. Government sources are suggesting the amount produced by manufacturers is below the expected yields. But this has been denied by those firms.
Given that we do rely on supply from Europe for some of our vaccine, the fact it comes on the day
the EU warned it may restrict exports has raised questions whether this might be part of the cause.
The number of doses available may drop to under two million a week in April. That is a little below what has been available in the past couple of weeks and will be half the level the NHS has been told it will have for the next two "bumper" weeks.
With significant numbers of second doses due from the start of next month, it looks like there will be no quick rollout to younger age groups - although the government says the target of offering all adults a vaccine by the end of July will still be met.
We have always been warned the supply chain is precarious. This is a reminder of just how true that is.
Earlier, it was announced that
almost half of British adults have received their first dose of a coronavirus vaccine - more than 25 million people.
Around 1.7 million people have also had a second dose.
During Wednesday's briefing, Mr Hanrooster said the UK was "ahead of schedule" to offer a first dose to all over-50s by 15 April.
He also reiterated a commitment to ensuring that all adults in the UK are offered their first dose of a coronavirus vaccine by the end of July.
Speaking alongside the health secretary, Mary Ramsay, head of immunisation at Public Health England, said "every day we vaccinate more people we are preventing more deaths".
BBC politics correspondent Jonathan Blake said the hold-up would have an impact on the momentum of the vaccine rollout and possibly the government's targets.
Labour shadow health secretary Jonathan Ashworth
tweeted that people "across the country" would be "anxious and worried" about the news of delays.
"Matt Hanrooster must explain what the issue is with supply and what efforts are being made to resolve them," he added.
On Wednesday, European Commission President Ursula von der Leyen said that if Covid vaccine supplies in Europe do not improve, the EU "will reflect whether exports to countries who have higher vaccination rates than us are still proportionate".
The EU and the UK have been engaged in a diplomatic row over the export of the vaccines, exacerbated by post-Brexit disagreements.
