In a letter to local vaccination sites on 17 March, NHS England primary care medical director Dr Nikki Kanani and vaccine programme lead Emily Lawson said: 'The government's vaccines task force has 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.
'They now currently predict this will continue for a four-week period, as a result of reductions in national inbound vaccines supply.'
The letter comes just weeks after vaccination sites were told that vaccine supply from 15 March would rise to double the level available in the first week of the month - and that this increased level would be 'sustained for several weeks'.
COVID-19 vaccine rollout
The letter warned that patients outside cohorts 1-9 identified by the Joint Committee on Vaccination and Immunisation (JCVI) should only be invited for jabs 'in exceptional circumstances' - and that those aged 49 years of age or below should only be invited if they fall into higher cohorts because they are clinically vulnerable, an unpaid carer or frontline health and care worker.
Vaccination sites have been told that the 'supply constraint' will mean that staff booked to work at some sites will need to be stood down or redeployed. The letter confirms that 'vaccination centres and community pharmacy-led local vaccination services should close unfilled bookings from the week commencing 29 March and ensure no further appointments are uploaded to the national booking system or local booking systems from 1 to 30 April'.
Speaking at a Downing Street briefing as the letter was published, health and social care secretary Matt Hancock said: 'Supplies are always lumpy and we are on course to deliver the offer that everybody who's aged 50 and above will be able to get vaccinated by 15 April, I recommit to that today.
'Supply schedules have moved up and down throughout this whole rollout, it's absolutely par for the course, and this is a normal operational letter.'
The health and social care secretary said the government also remained committed to 'all adults being able to get the jab by the end of July and we're on track to deliver on that commitment'.
Confirmation that the UK's vaccine supply is lower than expected comes as the government announced jabs had been opened up to patients aged 50-54 - the final group in JCVI cohorts 1-9.
More than 25m people UK wide have now received a first dose of vaccine - nearly four in five of the 32m total in cohorts 1-9.
NHS England's letter to vaccination sites called for a focus on reaching as many patients as possible in these groups alongside the delivery of millions of second-dose vaccinations that are due over the coming weeks.
The letter said: 'Now we have opened vaccination to cohorts 1-9, it is very important we focus our efforts on reaching as many of these groups as possible whilst administering second doses. We must take this time to deliver protection to the most vulnerable.
The letter added: 'Revised vaccine supply is likely to result in a reduction in workforce demand in hospital hubs and vaccination centres. We are asking systems to start planning now about how they will deploy staff to alternative settings to support increased cohort penetration.'