The Department of Health said it was essential for all children with asthma or diabetes — and those in other high-risk groups, including the over-65s and people with several chronic illnesses — to see their GPs for a winter flu jab. Approximately 1.1 million children suffer from asthma in Britain.

Scientists are concerned that, if the bird virus were to infect anyone already suffering from ordinary flu, the victim could then act as a “mixing vessel” in which the germ could adapt to spread more easily from person to person. This would be the key mutation that could trigger a devastating pandemic.

The possibility makes it more vital than ever for vulnerable groups to protect themselves against the three regular flu strains that will circulate this winter — even though the injection does not protect against the H5N1 avian flu that has killed at least 60 people in Asia.

It was confirmed yesterday that an outbreak of disease on a Turkish farm was indeed the H5N1 virus. Government veterinary specialists said they now assumed that the disease had also spread to Romania and confirmed that Britain was at risk.

There was panic in Whitehall yesterday when a news agency misquoted Markos Kyprianou, the EU Health Commissioner, as suggesting that all Euro-pean citizens should have a flu injection because of the risk from the avian virus. His advice was clarified as similar to the Government’s line — that only those most at risk should seek immunisation.

David Salisbury, the principal medical officer at the Department of Health, went on televison last night to defuse the alarm.

Fourteen million doses of vaccine against ordinary flu are available on the NHS for the over-65s and people with diabetes, asthma or chronic lung, kidney, liver and heart complaints, under the annual winter vaccination programme. Only about 71 per cent of those eligible had the jab last year, however, and officials said that a much higher take-up rate was now necessary to minimise the risk.

It is up to GPs to determine which patients are at risk of flu but, if people are not accepted, they can pay for a private prescription. The average cost of a flu vaccine is £25. Many companies in the private sector offer free jabs to their employees.

David Harper, the chief scientist at the Department of Health, said: “I would reiterate that it is very important that at-risk groups have the seasonal flu vaccine as normal.

“We have very successfully increased uptake among the over-65s, and we are now looking at the under-65s who would also most benefit, such as children with asthma, diabetes or renal disease.”

Seasonal flu vaccination would be important even if there were no risk from avian flu, but could also reduce the chances of the virus “reassorting” into a form that moves more freely between people.

“It’s important if you are likely to be exposed to the avian flu virus that you’re not already infected with seasonal flu virus, as if you are you can act as a mixing vessel for the viruses to swap genes,” Dr Harper said.

“If avian flu were to arrive here, then we would consider vaccinating poultry and farm workers against seasonal flu.

“There is no reason to vaccinate ordinary, healthy people. There is a concern that there is confusion about the different messages here.”

Debby Reynolds, the Government’s chief veterinary officer, also gave warning that the discovery of the deadly strain in Turkey increased the risk to Britain. She has ordered surveillance of all wild birds and poultry. She made clear, however, that the disease in Turkey was in birds and there had been no case in humans in Europe.

The Department of Health said that the confirmation of the highly pathogenic avian influenza strain in Turkey and a suspect outbreak in Romania was “of concern”.

It said: “It shows that there is a risk to the UK bird population and this is a developing situation which we are monitoring closely. However, hardly anybody is at risk of catching avian flu from birds.”

The last incident of avian flu in Britain was in 1992 — among turkeys on a Norfolk farm. It was a European variety and not the deadly Asian form.

Last night the Conservatives accused the Government of complacency and said that it was putting the country at risk. Andrew Lansley, the Shadow Health spokesman, said flu vaccines offered no protection against a bird flu pandemic.

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