The first sign that the avian flu virus H5N1 may be mutating into a form more infectious to humans has been reported by scientists. Researchers from the National Institute for Medical Research (NIMR) in Mill Hill, north London, have analysed viruses from two children who died of bird flu in eastern Turkey.

In one case, the analysis revealed mutations in the virus that made it more prone to infect humans. In a joint statement, Sir John Skehel, director of the institute, run by the Medical research Council, and the World Health Organisation, said a mutation had been traced in viruses isolated in Hong Kong in 2003 and in Vietnam last year.

"Research has indicated the Hong Kong 2003 viruses preferred to bind to human cell receptors more than to avian receptors, and it is expected that the Turkish virus will also have this characteristic."

The statement said the viruses were "very closely related" to H5N1 viruses in birds in Turkey, and also to viruses isolated last year in birds at Qinghai Lake in western China, a congregation point for migratory birds. The biggest fear is H5N1 will change into a form that can spread easily from person to person, triggering a global pandemic with the loss of millions of lives.

That could happen in a single big genetic "shift" involving a reassortment of the avian virus with human virus to create a strain of flu to which no one would have immunity. It could also happen in a series of smaller changes, known as "genetic drift", as the virus gradually evolves to become progressively more infectious to humans.

A spokesman for the MRC said the mutation observed did not amount to a major change. "The virus would have to change a lot more in other areas before it could cause a pandemic," he said. In the statement, the NIMR and the WHO say gene sequences of the viruses indicated they were sensitive to the antiviral drugs Tamiflu and amantadine. The British Government is stockpiling 14.6 million doses of Tamiflu. Earlier, Dr Bob McCracken, a former president of the British Veterinary Association, said the bird flu danger would be greatest during the migratory season for ducks. He told BBC Radio 4's Today programme: "The most likely place wild infected ducks are likely to land is in lakes and waterways." He said domestic birds should be kept away from those areas. Ducks can carry the virus without visible symptoms. The H5N1 virus has infected 150 people and killed at least 78 in six countries.