The British government's chief medical officer said a bird flu pandemic could result in 50,000 deaths in Britain.
Sir Liam Donaldson told BBC's Sunday AM programme: "In a normal winter flu year ... flu actually kills in excess of 12,000 people every winter with the normal winter flu.
"But if we had a pandemic, the problem would be that our existing vaccines don't work against it, we would have to develop a new vaccine, and people don't have natural immunity because it hasn't be around before.
"So the estimate we are working to in the number of deaths is around 50,000 excess deaths from flu.
"But it could be a lot higher than that, it very much depends whether this mutated strain is a mild one or a more serious one."
Sir Liam said the lessons of history strongly suggest that the bird flu virus will combine with a human flu virus, becoming easily transmissible.
"It has happened before. It happened in 1918, it happened in 1958, and it happened in 1968/69. These things come in natural cycles, once in a while, every 10 to 40 years the flu virus mutates into a strain which we haven't got natural immunity to. "
On the potential number of deaths, Sir Liam said: "If we are making comparisons with the Spanish flu in 1918/19, where 40 million people worldwide died, 250,000 in the UK, we are living in very different times. Those were in the days before proper hospital facilities, before intensive care, before antibiotics.
"We obviously have to be flexible in our plans and prepare for everything. But at the moment our estimate is that we expect around 50,000 to 55,000 (deaths)."
Sir Liam was asked whether he believes that the problem will hit this winter.
He said: "I think it is less likely that it will come this winter. The attention is focused in Europe because of these outbreaks. That doesn't mean that the pandemic flu is creeping closer to the UK, it simply means that bird flu is occurring in other parts of the world, as it has over the last five to six years.
"I think the likelihood is still that we will see the epicentre of this pandemic of flu, this mutation, in the Far East."
PA