The drug is Tamiflu, a prescription antiviral medicine thought to be the only protection available against a potential doomsday virus winging its way towards Britain.

Earlier this month a deadly bird flu, which has been spreading out from Asia, reached Russia. Yesterday a suspected case was reported in Finland, but the exact strain is not yet clear.

The H5N1 virus has killed tens of millions of animals, particularly chickens in Asia, but also other species. So far the strain has infected very few humans — only about 120 — but in those it has attacked it has been highly lethal. Half of them died.

The fear is that H5N1, like all flu viruses, will continue mutating and could turn into a strain that infects humans and passes from one person to another as easily as the common cold.

“If we do get human-to- human transmission, millions will die,” said Dr Nigel Higson, chairman of the primary care virology group.

“With huge numbers of people using air travel, it will move round the world very quickly. A large proportion of people in Africa would die. In western countries where we will hopefully have an avian flu vaccine and antivirals, 25% of the population will be infected and the death rate will be between 3% and 10% of the population. The fatalities will not just be the sick and elderly.”

Experts believe such a pandemic could cause a catastrophe on the scale of the 1918 Spanish flu that killed 50m people in 18 months.

Last month the Department of Health invited manufacturers to tender for a contract to develop and supply a vaccine against the strain. It is also spending up to £100m buying 14.6m courses of Tamiflu — an indication of how seriously it is taking the threat.

There is one big unknown: whether the virus can or will become transmissible between humans. Scientists have little evidence that it can do so at present and nobody knows whether it will be able to do so in the future.

“Although we expect this virus to become a pandemic we have no proof as yet that it will happen,” said Higson.

“To have a pandemic we have got to have a new virus.”

FLU is one of the most mutable viruses in the world, constantly shuffling its array of genes into new forms. The type known as H5N1 is thought to have originated in ducks from the Guangdong province of China in the late 1990s and drew particular attention because it proved devastating in poultry.

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