The prediction came from David Nabarro, a senior WHO expert on infectious diseases, who was appointed on Thursday as UN co-ordinator for avian and human influenza. He said the next pandemic could claim from five million up to 150 million lives.
Dr Nabarro called on political leaders to take immediate action to halt a human pandemic, and said that the higher death figure could result if governments failed to act now.
A WHO spokesman at the agency's Geneva headquarters made a surprise appearance yesterday at the UN regular media briefing in an effort to put Dr Nabarro's comments in context.
While he did not say the 150 million prediction was wrong, or even implausible, he said it was impossible to estimate how many could die. But he reiterated the WHO calculation that countries should prepare for 7.4 million deaths globally, arguing that was "the most reasoned position".
Scientists have made all sorts of predictions, ranging from fewer than two million to 360 million. Others have quoted 150 million. Last year, WHO's chief for the Asia-Pacific region predicted 100 million deaths, but until now that was the highest figure publicly mentioned by a WHO official.
Bird flu has been sweeping through poultry flocks and wild birds in Asia since 2003, killing millions, and has infected more than 100 humans, of whom more than 60 have died. This has proved that the strain of avian flu circulating in the Far East - H5N1 - is lethal to humans, with a death rate of more than 50 per cent.
The fear is that the virus may mutate so it becomes easily transmissible from human to human, triggering a lethal pandemic that would spread around the world.
Ordinary winter flu, which causes outbreaks in Britain and elsewhere each year, is one of the most infectious diseases known, and spreads rapidly among populations. If avian flu were to acquire the same level of infectivity, nothing would halt its spread around the globe.
In the UK, Sir Liam Donaldson, the Government's chief medical officer, said it was a "biological inevitability" that the next flu pandemic will cause serious harm to the health of people in Britain.
Tens of thousands of people will die in the pandemic which could strike as early as this winter or not for another decade, he warned. It was not a question of whether it would strike but when, Sir Liam said.
On BBC Radio 4, he said that contingency plans for the UK had not changed and were still based on 50,000 possible deaths. Asked if the UK was ready to face the threat, he said: "I don't think I would ever want to be as bold as to claim that."
He added: "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 very serious impact on the health of our country. That's a biological inevitability."
The UK has ordered 14.6 million doses of Tamiflu, an antiviral drug that cannot prevent avian flu but can lessen the symptoms, of which 900,000 doses have so far been delivered.
"Those won't eliminate the problem but for people who get it, it should reduce the severity of their attack and it should prevent many people from dying," Sir Liam said.
WHO officials say the only hope of halting the next pandemic would be to snuff it out at the start by detecting an outbreak early and treating the 20,000 people closest to the centre of the outbreak to prevent its spread.
The WHO is stockpiling three million doses of anti-viral drugs to be flown to any part of the world to be used in that eventuality.Reuse content