Britain's leading expert on the influenza virus is calling on the Government to stockpile millions of doses of anti-viral drugs to protect the population against a potential global epidemic triggered by avian flu.

Sir John Skehel, director of the National Institute for Medical Research, said the spread of avian flu through Vietnam, South Korea, Japan, Thailand and Cambodia, where six people have died and millions of chickens have been infected, was unprecedented.

The European Union banned imports of poultry meat from Thailand yesterday as countries around the world stepped up measures against the threat of a pandemic. Britain is the second largest importer of Thai poultry meat and products in the EU after Germany. Thailand has the world's fourth largest chicken industry and is the second largest exporter to the EU after Brazil.

Experts fear that as the virus spreads in the bird population and the number of people exposed to it grows, the chances increase of a lethal mutant form developing with the potential to spread from human to human.The outbreak of bird flu sweeping through the Far East has claimed half a dozen lives and caused alarm through out the region.

All those infected in the Far East outbreak caught the disease directly from birds but Sir John warned that if the virus mutated and acquired the capacity for human-to-human transmission the impact could be devastating. He said: "The more frequent these infections in humans become [caught directly from birds] the greater the chance of a mutation that will allow the virus to transmit to other humans. It is a difficult risk to predict. It doesn't look that easy for it to get a hold in humans. But when it does it is so devastating."

In autumn last year, an outbreak of avian flu in the Netherlands led to the slaughter of 25 million chickens and the destruction of 100 million eggs. Some 80 vets contracted eye infections caused by the virus and one, who was not taking anti-flu drugs, died from a respiratory infection.Another vet transmitted the virus to his family.

Sir John said the Netherlands outbreak was caused by an H7 strain of the virus which appeared to be harder for humans to catch than the H5 strain responsible for the current Far Eastern outbreak.

An H5 strain was also responsible for the 1997 outbreak of avian flu in Hong Kong which infected 18 people, of whom six died.The death rate of one third is equivalent to that for smallpox. "It is a horrible thing," Sir John said.

Asked how Britain should be preparing for a possible flu pandemic, he said: "There should be a readily available supply of anti-influenza drugs. The impact of an epidemic if there were sufficient anti-virals could be decreased."

The last global epidemic of flu in 1968 was caused by the mixing of an avian and human flu virus in a person which resulted in the avian virus acquiring the capacity for human transmission in a process known as reassortment. The result was a virus to which the human race had no immunity. Sir John said: "The thing that has changed since 1968 is that today we have anti-viral drugs. The question is, are there sufficient to protect the population?"

Two anti-flu drugs have been licensed in the past five years - Relenza, made by GlaxoSmithKline, and Tamiflu, made by Roche. Neither drug can prevent the illness but research shows they can shorten its duration and lessen its severity, potentially saving lives.

Asked if Britain should be stockpiling anti-virals, Sir John said: "I don't think there is any doubt about that. It would take a reasonably long time to make a lot of anti-virals which is why the Government should be stockpiling them."

Anti-flu drugs can be used against any strain of flu. They are different from vaccines, which can prevent infection with specific strains, but take months to produce. Research is under way into an avian flu vaccine but there would be difficulties in producing it in sufficient quantity and fast enough to halt a pandemic.

Alan Hay, director of the World Health Organisation influenza monitoring centre at Mill Hill, north London, said: "There are moves to stockpile anti-flu drugs. At present the stockpile is quite small. The likelihood of the virus acquiring the ability to spread from one person to another is low. But this is a major concern."

A spokesman for the Department of Health said: "The UK has previously stockpiled drugs for flu, to a limited extent. The need for additional stockpiles is under review." Research was continuing into developing a vaccine, he said.