Claims that the UK may be unprepared if this year's flu season turns into "the big one" have been dismissed by the Department of Health.
Flu pandemics claimed millions of lives
US researchers writing in the journal Science says that many countries may be vulnerable to a global pandemic.
They say that vaccines and drugs should be stockpiled to guard against a severe outbreak - but this is not happening.
A spokesman for the department said that the UK is "heavily committed" to preparing for a major flu epidemic.
Richard Webby and Robert Webster, from the Division of Virology at St Jude's Children's Research Hospital in Memphis, said that an influenza pandemic was inevitable - and may be imminent.
Pandemics in 1918 - the so-called Spanish flu - 1957 and 1968 killed many millions.
Recent years have seen particularly mild flu in the UK, and there is nothing so far to suggest that this year's "Fujian" strain will pose more problems than average, even though it is more virulent in children.
However, the researchers say that, should a deadly new strain emerge, the only way of protecting the public would be through mass vaccination - and the sheer quantities of vaccine required would be huge.
In the meantime, the progress of the illness would have to be checked by antiviral drugs.
However, this means that substantial quantities of these drugs would have to be stockpiled.
The researchers say that no country, including the US and UK, has been stockpiling anti-flu drugs for this purpose.
"Pandemic influenza has already threatened twice in 2003.
"The events associated with these outbreaks show that we are in a much better position to respond rapidly to an influenza threat than we were in 1997.
"However, much remains to be accomplished. Overall, our state of preparedness is far from optimal."
Even if antivirals are available, they say, there would have to be the capacity to manufacture large quantities of vaccine as soon as one was developed. This is not currently present, they say.
"The world will be in deep trouble if the impending influenza pandemic strikes this week, this month or this year.
"It is now time to progress from talking about pandemic vaccines to taking action."
A spokesman from the Department of Health said that these issues were being addressed.
"The UK is heavily committed to improving preparedness for an influenza pandemic.
"We have previously stockpiled drugs for flu, to a limited extent, and we are currently reviewing a possible need to do this with antivirals.
"Improving the processes of getting a vaccine to protect against a flu pandemic is something the world scientific community, led by the World Health Organization, is addressing."