The Government is making a mistake by stockpiling only one antiviral agent in preparation for a flu pandemic, experts said.
Tamiflu is the Department of Health's weapon of choice against the H5N1 bird flu strain which potentially could kill millions of people around the world.
But a report by the Royal Society and the Academy of Medical Sciences warns that Tamiflu on its own may not be enough.
Scientific evidence is emerging that the flu virus can develop resistance to the drug, said the experts.
Stocking an alternative antiviral drug - Relenza - alongside Tamiflu could provide a valuable second line of defence, they said.
Sir John Skehel, vice-president of the Academy of Medical Sciences, who chaired the working group which produced the report, said: "We are concerned that decisions are being made, as the UK prepares for a possible pandemic, that fail to take account of expert advice.
"For example, the decision to continue to stockpile just one antiviral drug is a major concern. This needs to be reconsidered.
"New evidence that H5N1 can develop resistance to Tamiflu indicates that a combination of antivirals should be stockpiled by the UK for the most effective management of a pandemic.
"The Government was right to order Tamiflu in early 2005. However, we are concerned that it is not updating its plans as the landscape of what we know about influenza changes."
H5N1, which emerged in eastern Asia, has already claimed an estimated 153 human lives as a result of contact between people and birds. However, experts believe it is only a matter of time before the virus mutates into a form that can easily be transmitted from person to person. A pandemic would then result which could lead to millions of deaths.
