Thousands of medical laboratories in the US and around the world are racing to destroy a pandemic flu virus, sent to them as part of routine testing kits, in an incident that again underlines the risk of disease from laboratory accidents.

The World Health Organisation said on Wednesday that nearly 3,750 laboratories, all but 75 of which were in the US, had been told to destroy samples of the deadly H2N2 virus received in proficiency testing kits sent out by the College of American Pathologists.