Patients may have to wait to receive their annual flu jab this autumn because of manufacturing problems.
Patients told to wait until alerted to come for the jab
Doctors have been told to prioritise the likes of pensioners and those with heart and chest conditions when the immunisation programme begins.
But the government said the delay was unlikely to lead to a flu outbreak.
Manufacturers should know the full extent of the problem - particularly the impact on total quantity of vaccine available - by the end of July.
Patients have been told to wait until their clinic invites them to go for their seasonal flu vaccine.
Flu is estimated to kill several thousand people in the UK each year. Vaccines help prevent deaths but have to be made afresh each year because the flu virus mutates.
The Department of Health said manufacturers are currently encountering problems growing one of the vaccine virus strains recommended for this year's seasonal flu vaccine.
Flu is estimated to kill several thousand people in the UK each year
10-15% of the population develop flu each year
100,000 flu particles can be projected into the air with just one sneeze
In 12 hours, the flu virus can invade 1 million nose and throat cells
It comes after flu jab supplies ran low last year because of a surge in demand - put down to fears over bird flu.
It meant many of the most vulnerable people did not get their jab until well into winter.
A Department of Health spokesman said: "This means that although the Department of Health has reserved more vaccine for the UK for 2006/7, some supplies of flu vaccine will be distributed later than usual."
The Department of Health predicts 15.2 million vaccines are needed.
Mr Paul Rayner, of the UK Vaccine Industry Group, said: "We're hoping by the end of July to be able to confirm we can deliver these vaccinations."
Dr David Salisbury, director of immunisation at the Department of Health, said: "There are two messages that we need to get across, the first is the primary care and that is 'you need to be aware there may be a delay in your vaccinations coming and we want you to use your patient registrar to match the priority patients against you supplies.'
"The second message is to the public and that is 'we are hopeful that all of the vaccines will come but you should wait until you are alerted by your practice that they have the vaccination for you'."
He added that an outbreak of influenza was unlikely unless vaccination supplies were really late and the flu season arrived exceptionally early.