The flu virus is becoming resistant to key drugs which are used to treat it, a study suggests.
Some experts believe a flu pandemic is overdue
Doctors in Japan carried out tests on 50 children who were admitted to hospital with flu in 2002 and 2003.
They found that nine children became resistant to the new generation of drugs used to fight the virus.
The findings, published in The Lancet, have sparked concern because these drugs would be needed if there were a flu pandemic.
The medicines belong to a class of drug called neuraminidase inhibitors.
They include Relenza, which is manufactured by GlaxoSmithKline, and Tamiflu, which is made by Roche.
They are currently the most effective weapons against flu. They stop the virus from escaping from one cell and infecting a healthy cell.
Scientists have already known that some strains of flu are resistant to these drugs.
However, this latest study suggests resistance could be much more common than previously thought.
Dr Maki Kiso and colleagues from the University of Tokyo said the findings could have serious implications if there is a major outbreak of flu.
"Our results suggest that a higher prevalence of resistant viruses should be expectant during pandemic control efforts using neuraminidase inhibitors," they said.
Some experts believe that a flu pandemic is overdue. There were three pandemics in the last century. The biggest one in 1918 is thought to have killed 40m people.
Dr Anne Moscona from New York's Mount Sinai School of Medicine has called for urgent research into the issue.
"We need more information on the emergence of resistance," she said.
"We urgently need to know whether resistant variants, such as those identified in Kiso's study, are transmissible."
John Oxford, professor of virology at Queen Mary's School of Medicine, said the findings were a wake up call.
"This study suggests that resistance is higher than we previously thought," he told BBC News Online.
But he said research into these resistant strains did not find any evidence to suggest that they can infect others.
"They are crippled in some way. While they can mutate, they cannot spread."
But he added: "It is a wake up call. We must continue to monitor these viruses and learn more about them."