Professor Dame Sally Davies, the Government’s chief medical officer, has written to all GPs and NHS hospitals warning of a “sharp increase” in the numbers of patients admitted to intensive care because swine flu has caused their lungs to fail.

The letter says pressures on critical care services are “significantly over and above” those expected at this time of year.

Crucially, the memo, written on Tuesday, says that while “half of patients requiring respiratory support have had recognised comorbidities [underlying health problems] which increase the risk for severe influenza, half have had no recognised comorbidities.”

A spokesman for the Department of Health (DoH) confirmed that the presence of so many previously healthy people among those worst affected by the virus was “unusual” and said anyone concerned about worsening flu-like symptoms should contact their GP.

While overall flu levels remain normal for this time of year, the rate of flu has more than doubled in just seven days, latest figures show.

The official death toll from flu this winter has now reached 17, including six children. Fourteen of the deaths were linked to swine flu, and none of those who died had been vaccinated against the virus.

In the same letter, the senior doctor says pregnant women, children, young adults, and those who are overweight are among the most severely ill.

Yesterday, Prof Davies criticised the Government’s decision to axe the annual publicity campaign urging people in at risk groups to have the seasonal flu jab, which protects against the main viruses in circulation, including swine flu.

Prof Davies said: “The Government took a decision not to use the marketing campaign this year. I would prefer to have done it because I believe in belt and braces. I am a doctor – I don’t like leaving things to chance.”

For the first time, the winter flu jab is being offered on the NHS to pregnant women, who were disproportionately affected by last year’s swine flu outbreak, as well as to elderly people and those with existing health problems.

Pregnant women are being advised to be immunised, no matter how far into their pregnancy they are, since they are thought to be more than four times more likely than others to develop serious complications if they contract swine flu.

The medical officer said the number of people seeing their GP because of flu was “bang in the range” for the season.

However, the sharp rise in all flu cases in seven days and the number of people below the age of 65 being admitted to hospital were reasons why the Government was examining the patterns carefully, she said.

GPs are worried that elderly people, who suffered less than younger people in last year’s swine flu pandemic, are failing to protect themselves against other strains of flu.

Vaccination rates among the elderly are down two per cent on last year, while among at risk groups, rates are down by 6 per cent.

In some parts of the country, pharmacists and hospitals are complaining of shortages of the drug used to treat the virus. Chemists and hospitals in Yorkshire were unable to track down supplies of Tamiflu, with NHS bosses describing “difficulties with the national supply chain”. The DoH said there was no national shortage of the drugs, and that any local delays “should be short-lived”.

Last night a pregnant woman was fighting for her life, after being put in a medical coma to save her and her unborn child.

Fallon Devaney, 25, from Derbyshire, a mother of four who is five months pregnant, was in intensive care in Nottingham’s Queen’s Medical Centre, after being admitted to hospital struggling to breathe.

Miss Devaney was taken to hospital on Tuesday after flu symptoms left her struggling to breathe. Since then, doctors have induced a medical coma, in an attempt to preserve her strength as her body battles the virus.

Her mother, Linda Fearney, said her daughter had not had the annual flu jab, and urged others in “at risk” groups to be vaccinated. she said: “She’s near death and it’s touch and go. We do not know if she’s going to survive.”

At least four more pregnant women with swine flu in England have undergone intensive care treatment. in Scotland, a woman with swine flu has been placed in a coma after giving birth to a daughter by caesarean section.

A spokesman for the DoH said GPs would be contacting patients in “at risk” groups to invite them to have the vaccination. he said: “It is vital that those at greatest risk are vaccinated as soon as possible.”

Anyone who did not know if they were in such a group should contact their GP, he said.