The UK would have just weeks to prepare for a flu pandemic, such as a mutated avian flu virus, if it started to spread rapidly in Asia, experts warn.
People in close proximity to birds have been worst affected
The Health Protection Agency (HPA) put together statistical models to estimate how soon the UK would be affected.
Scientists say measures such as travel bans and airport screening could slow the spread of the disease.
But they say once it is here there will be little that can be done, as the disease spreads so easily.
Experts repeatedly warn of the danger a flu pandemic could occur.
They suggest the strongest possibility is that bird flu, which has infected at least 100 people and killed more than 50 in South Asia, could mutate and combine with a human flu virus, which would mean it could spread between people very easily.
The H5N1 strain of avian flu in humans has a high fatality rate.
'Not much time'
Dr John Edmunds, from the HPA's centre for infections, said that once clusters of flu in Asia started to get bigger, with more people infected, the alarm would be raised.
He told the HPA's annual conference at the University of Warwick any pandemic would spread more quickly around the world than the last one, which occurred in 1968/69, when the northern hemisphere was affected during their winter, and countries in the southern hemisphere in their winter.
Dr Edmunds said: "This time we would expect it to be spread more quickly than it did last time because we have a lot more flights and there's a lot more contact between people these days.
"If it's spreading widely in south east Asia, then a few weeks is really all we could expect before the pandemic arrived here.
"So there isn't much time."
But he said analysis showed travel restrictions were likely to buy very little time, and be very expensive.
Screening people coming into the UK would be "very ineffective", and likely to pick up few cases because during a flu pandemic, people with symptoms would not be allowed to board a plane anyway, he said.
But many of those who did travel could be incubating flu but not showing any signs, meaning screening, say for high temperatures, on arrival in the UK would not stop cases getting through.
Dr Edmunds said: "There is only one thing that can be done to stop it, and that is to stamp it out at source and if you're lucky you can stop a pandemic developing at the source."
The UK government, like others around the world, is already stockpiling antiviral drugs.
It has ordered around 14.6 million courses of the drug Tamiflu - enough to treat around a quarter of the population.
The drug reduces the severity of flu symptoms and can also mean the length of illness is shortened.
The government is also planning to buy vaccine to protect against H5N1.
But experts will not know the make-up of any pandemic strain until it appears, and it can take six months to develop a vaccine against a particular strain.
Dr Nigel Lightfoot, the HPA's director of emergency preparedness, said the government's planning had been "rigorous".
He said public information campaigns would be crucial in the weeks before a pandemic arrived in the UK.
And he added: "The public information campaign has already started and that needs to continue, but we don't want to frighten everybody to death.
"People should know that the best thing they can do if they feel ill is to stay at home. Don't try and be a martyr and go to work."