It is a "biological inevitability" that a flu pandemic will hit the United Kingdom badly, England's chief medical officer has warned.
Sir Liam Donaldson says anti-viral drugs are being stockpiled
Sir Liam Donaldson's comments came after the head of the UN's team tackling bird flu claimed a pandemic could kill up to 150 million people.
Contingency plans are based on 50,000 Britons dying from flu, he added.
But he said the figure, first announced in March, could rise and would not say the government was fully prepared.
"I don't think I would ever want to be as bold as to claim that," he told the BBC.
Sir Liam said: "It's inevitable that when the flu pandemic comes...it will have a very serious impact on the health of our country."
Earlier David Nabarro, the UN's new co-ordinator for avian and human influenza, said it was likely that strains of avian flu currently affecting Asia could mutate to infect humans.
Sir Liam said it was impossible to predict when the pandemic could reach Britain, saying it could arrive next winter or in 10 years' time.
He also explained that it was difficult to build up supplies of vaccines because the drugs can only tackle specific strains of the virus.
Instead anti-viral drugs were being stockpiled in anticipation of the disease reaching the UK.
"Those won't eliminate the problem but for people who get it [flu], it should reduce the severity of their attack and it should prevent many people from dying."