There are fears that humans could catch the flu from close contact with birds infected with the H5N1 virus and trigger a pandemic.

The Government has already built up enough stocks of the drug Tamiflu (oseltamivir) to treat a quarter of the population.

Tamiflu is a neuraminidase inhibitor, but experts say that governments should also be stocking another type of drug, known as ion channel inhibitors.

Although effective against several strains of flu viruses, their side effects mean they are not being widely stockpiled, the experts said.

The drugs are also linked to a rapid build up of resistance.

Sotirios Tsiodras, a lecturer in infectious diseases at the University of Athens Medical School, and John Mooney, a public health specialist trainee at the South West Public Health Training Programme in Dorset, said that combining the two types of drugs may actually reduce side effects and resistance.

Writing in the British Medical Journal (BMJ), they said that just one mutation could lead to a flu strain resistant to neuraminidase inhibitors or ion channel inhibitors.

But in laboratory tests, combining the two reduced resistance and even the emergence of resistant strains of the H5N1 virus.

The authors write: "Importantly, the combination of ion channel and neuraminidase inhibitors in vitro reduced the emergence of resistance and may even act synergistically against influenza A viruses. Combination therapy may also allow the use of a lower dose of ion channel inhibitors, which is known to reduce side effects."