Senior GPs said today they are "deeply concerned" that a major flu crisis is brewing.
Fewer patients in at-risk groups - including pregnant women and those with diabetes and heart disease - are coming forward for the seasonal flu jab.
The vaccine protects against swine flu, which is circulating again this winter, as well as other types, including flu type B.
Even those people in at-risk groups who had the swine flu jab last year should come forward for the new seasonal vaccine, according to the Department of Health.
Today, the British Medical Association (BMA) said lower immunisation rates have increased fears that a normal seasonal flu outbreak could prove much more serious this time.
An estimated 67.2% of over-65s and 41.5% of under-65s in at-risk groups have had the flu jab in England.
This is just behind last year's figure for the over-65s, but 5% behind vaccination rates seen this time last year for at-risk groups.
Chairman of the BMA's GPs committee Dr Laurence Buckman has written to the Government urging it to step up its publicity campaign.
He said: "Family doctors are already seeing high rates of influenza and they have been telling us that they are also seeing a lower uptake than usual for seasonal flu immunisation.
"Myths persist about the safety of the vaccine, especially after swine flu.
"The vaccine has been thoroughly tested and we strongly urge patients to make an appointment with their GP and get vaccinated."
He added: "In a year when the Government is hoping to make efficiency savings, leaving a larger proportion of the population vulnerable to contracting influenza risks far more serious spending implications in primary and secondary care.
"GPs will do what they can but we would urge the Government to step up their publicity to highlight the safety and effectiveness of flu jabs."
Figures out yesterday revealed 17 people have died from flu so far this winter.
The Health Protection Agency (HPA) said 14 deaths were from swine flu and another three from flu type B.
Of those who died, all were aged under 65, with six deaths among children under 18.
At least eight of the 17 people was in an "at-risk" group. None was pregnant and none had been vaccinated.
Overall, 17 people, including four pregnant women, with confirmed swine flu have received specialist intensive care treatment - known as extra corporeal membrane oxygenation (ECMO) - so far this flu season.
ECMO is for the most severe cases and uses an artificial lung to oxygenate the blood outside the body.
Data also showed that the number of people visiting their GPs with flu-like illness has risen to 35 cases per 100,000, up from 13 in the previous week.
The highest rates are among five to 14-year-olds and babies aged one to four.
At-risk groups include people over 65, those suffering from a chronic heart or chest complaint, people with asthma, chronic kidney disease, diabetes, and those with lowered immunity due to cancer.
Professor Dame Sally Davies, chief medical officer for England, said today: "It is particularly important for all pregnant women, no matter what stage they have reached in their pregnancy, to get their flu vaccine as soon as possible.
"This year, pregnant women are being offered the flu vaccine free of charge, regardless of their stage of pregnancy.
"Pregnant women are more likely to become seriously ill if they catch flu, which is why it is particularly important for them to get their jabs."
Dr Katie Laird, senior lecturer in pharmaceutical microbiology at De Montfort University, said: "Due to the nature of the flu virus - where mutations occur readily - it is difficult for the body's immune system to recognise the infection and can leave individuals such as the young, elderly, pregnant women and those with compromised immune systems without much natural immunity against the virus.
"Therefore, it is essential for at-risk groups to consult their GP on whether vaccination is suitable for them."
June Davison, senior cardiac nurse at the British Heart Foundation, said: "If you have heart disease then you have a greater risk of becoming more seriously ill from flu than the general population. So if you have heart disease, it's very important to have the flu jab to protect yourself against this infection."Reuse content