Thousands of vulnerable patients could have to wait up to two weeks to receive their flu jab because of a manufacturing delay.
Pregnant women and people aged between 16 to 65 who hoped to be immunised next month face being turned away.
Stock is being held up due to manufacturing problems at Sanofi Pasteur, one of the top NHS suppliers of flu jabs each winter.
The company has written to GPs in England, alerting them of the situation ahead of the incoming flu season.
Thousands of patients may have to wait up to two weeks to receive their flu jab due to manufacturing problems, NHS supplier Sanofi Pasteur warned
Sanofi Pasteur supplies around a third of the flu jabs to GP practices every year in England, according to estimates.
Almost 14million at-risk people in the UK took their free NHS jab last year, including more than 11million in England alone.
This includes over-65s, people with weakened immune systems, and expectant mothers.
The jab can take up to three weeks to kick in and protect you from catching the flu, and is necessary for saving lives. It can cause severe illness and even death among vulnerable people.
Some 8,000 people in England die from the flu every year, of whom 6,000 have a serious problem with their heart or lungs.
Sanofi Pasteur said 40 per cent of surgeries will not receive planned supplies on October 7 – the first week of deliveries, according to The Guardian.
Around 25 per cent of surgeries will need to wait until a week later, and 15 per cent will get their deliveries two weeks later, from 21 October.
The second batch of deliveries, which were supposed to be received by October 14, will be held back by one to two weeks, the letter said.
The letter said ‘100 per cent’ of second deliveries will be delayed.
Hospitals, who also give the jabs out to their patients, will also face a shortage, although on a much smaller scale.
Delivery of the third and final supplies, due at hospitals from 4 November and GP practices from 18 November, ‘remains unchanged’ at the moment, the letter said.
Those affected by the delay are supposed to be receiving the quadrivalent flu vaccine, a jab that is designed to protect against four different flu viruses.
Over-65s are not affected by the delay. They receive the trivalent vaccine, supplied either by Sanofi or Seqirus – who said GP practices will receive all deliveries.
Pharmacies, including Superdrug and Boots, are not believed to be affected. They offer the vaccine free for at-risk groups or for a price of £9.99 and £12.99 for everyone else.
Brexit has stoked fears that medicine supplies could be disrupted, with drug giants in the dark about how medicine will be transported into the UK in the event of a No Deal.
But it is understood the hitch in flu jab supplies are to do with manufacturing and packaging problems, and not Brexit.
Sanofi Pasteur has not clarified what the manufacturing delay is.
This is the second delay to supplies of Sanofi’s quadrivalent influenza vaccine that it planned to deliver to the NHS this autumn.
The first hold-up occurred in July, after the World Health Organization took longer than expected to decide which strains of flu would circulate this winter.
They said influenza vaccinations have become increasingly difficult to manufacture due to the viruses frequently changing.
The organisation also warned countries globally to prepare for a potential scenario of four-week delay.
NHS bosses were urged to step up anti-flu preparations earlier this year after a stark rise in the number of virus cases during winter in Australia.
Top British doctors said the increase in Australia could be a sign of what is to come in the UK this winter.
Dr Nick Scriven, president of the Society for Acute Medicine, said: ‘One concern is that last year we saw predominantly one strain of influenzas but already there are reports of two or three strains significantly circulating in the population that could complicate vaccination.’