Government plans to tackle a predicted bird flu pandemic have been attacked by a scientist who fears an outbreak could kill two million people in the UK.
Experts say the bird flu will mutate with a human flu
Professor Hugh Pennington, president of the Society for General Microbiology, criticised ministers' "optimism" and said a vaccine needs to be ordered now.
Experts say bird flu could spread fast among humans and lead to a pandemic.
The government says its plans to tackle an outbreak - which include stockpiling
antiviral drugs - are comprehensive.
Bird flu has killed at least 47 people in South East Asia over 15 months and there are suspected cases of the virus being passed between humans.
The World Health Organization (WHO) fears 100 million people could be killed worldwide - the 1918 pandemic killed 50 million, including more than 200,000 in the UK.
'No stopping it'
Professor Pennington, who says the 2 million death toll is an estimate, said: "They (the government) are being very optimistic about how they see it developing over the next year or two.
"We know that the virus, when it gets into people - which doesn't happen very often but has happened in the Far East - is very, very lethal, much more than the ordinary kind of flu virus that we're used to," he told BBC News 24.
"If it does that and it keeps this ability to kill, where it kills between 60% and 80% of the people it infects, that's where these big numbers of deaths come from."
In an interview with the Independent on Sunday, he said the government hoped the problem would "go away" and likened its attitude to the BSE crisis.
Former health minister Edwina Curry, forced to resign after warning of the risk of salmonella in eggs, said the government should listen.
"If you are going to protect people you need to do it now," she said.
"If I were the health minister I would have a meeting tomorrow morning and I would be vaccinating people, for example, in the health service by the end of the week. I would rather be ready."
However, BBC medical correspondent Fergus Walsh said Professor Pennington's estimation could be too low.
He said, based on WHO estimates that one in four Britons could become infected and a mortality rate of 75%, the figure would be about 11 million.
The government says vaccines cannot be prepared for any human-to-human outbreak as it is not known what form the virus would take and how it would mutate.
But Prof Pennington said the structure of the virus had not changed a great deal in recent years and countries such as the US were already ordering supplies.
He said that while it may never be needed, Britain could get stuck "in a queue" for a vaccine if it waits too long.
Chief Medical Officer Professor Sir Liam Donaldson has previously said: "The plan set out recently outlines many other steps which will be needed to reduce the pandemic's impact.
"The NHS and the government are taking steps to ensure we are as well prepared as we can be to cope with this."
'High quality plan'
As well as ordering 14.6 million doses of antiviral drug - enough for a quarter of the population - the Department of Health action plan includes quarantine measures and arrangements for the emergency services.
Dr Anarfi Asamoa-Baah, assistant director general for communicable diseases at the WHO, said: "From an initial review of the document, I note that this is a high quality plan.
"It shows that the UK continues to be at the forefront of preparations internationally for pandemic influenza."