Researchers have found a change in the H5N1 bird flu virus which makes it resistant to the antiviral flu drug Tamiflu.
Scientists at the Medical Research Council (MRC) found the mutation in human cases of the disease, prompting concerns that a single drug approach may not be effective in the event of a bird flu pandemic in humans.
The study at the MRC's National Institute for Medical Research looked at a mutation in the structure of the flu neuraminidase (N1) that has been observed in human cases of H5N1 and in seasonal flu.
N1 is the target for both Tamiflu and Relenza, the two existing flu drugs, which aim to stop the N1 releasing the virus from infected human cells and allowing the disease to spread.
The researchers found that when mutation occurred, the virus became resistant to Tamiflu, while still remaining susceptible to Relenza.
"What this research shows is that stockpiling any one drug to prepare for a potential H5N1 pandemic is unlikely to provide adequate cover," said Dr Steve Gamblin, who led the research, the results of which are published in the journal Nature.