A British flu pandemic is inevitable and will put the lives of tens of thousands of people at risk, according to the country’s top medical officer.

Sir Liam Donaldson said today that it was a "biological inevitability" that when the pandemic arrives, it would have a "serious impact".

The Chief Medical Officer for England said that the government’s contingency plans were looking at 50,000 deaths in the UK from the pandemic. He added that a pandemic could arrive at any time and that it was impossible to state that Britain was ready to cope.

Experts have said that avian flu will eventually mutate so that it can spread easily between humans, leading to a pandemic strain.

The H5N1 strain of Avian flu has killed over 60 people in south-east Asia in the last two years including, it is believed, five people in Indonesia who came into contact with affected poultry in the last month. The disease is believed to have been transmitted through the droppings or saliva of sick birds.

Dr David Nabarro of the World Health Organisation - which has already said a pandemic is only a matter of time - said today that a pandemic could kill between five million and 150 million people worldwide, depending on action taken now.

Questioned on what Britain was doing to prepare for the pandemic, Sir Liam Donaldson told BBC Radio 4’s The World at One that anti-viral drugs were being stockpiled. It is understood that about 14.6 million courses of Tamiflu - enough to protect a quarter of the population - have been stored.

Sir Liam added however: "Those won’t eliminate the problem but for people who get it, it should reduce the severity of the attack and it should prevent many people from dying."

Asked if he thought Britain was ready to face a pandemic he replied: "I don’t think I would ever want to be as bold as to claim that.

"It’s inevitable that when the flu pandemic comes, and we don’t know whether that will be next winter or even in five or 10 years’ time, that it will have a serious impact on the health of our country. That’s a biological inevitability."

Health authorities believe that if the bird flu virus mutates in Asia and cases start spreading, Britain will only have weeks - if not days - to prepare.

Travel restrictions, airport screening and other strategies would be of little use to stop cases multiplying rapidly, according to statistical models carried out by the Health Protection Agency.

Last month the experts from the European Union’s 25 member states met in Brussels to coordinate contingency plans to combat the threat. Their worries had mounted after the H5N1 strain was found to be moving westwards and had reached Siberia.

Amid fears that migrating birds arriving from the east could be carrying the virus, German and Dutch poultry farmers complied with government orders to move all their birds indoors.

After the recent outbreak in Indonesia, the United Nations warned its government to start a mass slaughter of poultry in affected areas.

Indonesian health minister Siti Fadilah Supari said last week it was an epidemic adding that there would be more deaths unless they could pinpoint the source. She later toned down her comments but the government has declared it an "extraordinary situation". It has given itself the powers to confine and forcibly treat anyone showing bird flu symptoms.

Australia has given the government 10,000 doses of Tamiflu but there are concerns there is not enough of the drug being produced.

About 300 million doses of vaccine are produced annually, but demand still exceeds supply. Flu affects between 5 per cent and 15 per cent of the world's population, resulting in between 3 million and 5 million cases of severe illness and 250,000 to 500,000 deaths each year.

First discovered in China in 1997, bird flu has infected more than 100 humans since 2003. It has a high mortality rate, with more than 60 dead in Vietnam, Thailand, Cambodia and Indonesia.

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