An Achilles’ heel of the flu virus has been identified by scientists, suggesting a new target for drugs that are effective against many of its different strains.

Research has shown a critical weakness in a key flu protein that is common to all the most dangerous varieties of the virus, including H5N1 avian flu and the strains that infect millions of people each winter.

The findings offer a fresh approach to designing antiviral drugs that could be used to control seasonal flu epidemics and to fight the potential emergence of a pandemic strain.

Though two good antiviral agents, Tamiflu and Relenza, are already available and a third, peramivir, is in clinical trials, the development of more options for treating flu is considered important to global preparations for a pandemic.

A new drug would be particularly valuable if a pandemic strain of flu were to evolve resistance to Tamiflu, the front-line treatment that has been stockpiled by many countries, including Britain.

Some versions of the H5N1 virus, which has infected 258 people and killed 154, have already shown resistance to Tamiflu, and if such a strain became dominant the drug would become useless. Last month an expert report by the Royal Society warned the Government not to rely on it exclusively.

The new drug target has emerged from research in the United States, which has for the first time unravelled the structure of the flu nucleoprotein, known as NP. The findings are published in the journal Nature.

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