The head of the World Health Organisation hit back at critics who have accused it of over-reaction to the swine flu crisis, warning it may return “with a vengeance” in the months ahead.

In her first extensive media interview since alerting the world to a potential flu pandemic nine days ago, Margaret Chan, the agency's director-general, told the Financial Times that the end of the flu season in the northern hemisphere meant an initial outbreak could be milder but then a second wave more lethal, as happened in 1918.

Fresh data from Mexico suggested the impact of the flu could be less than initially thought. José Angel Cordova, health minister said the flu virus epidemic had passed its peak and was declining. “The evolution of the epidemic is now in its phase of descent,” he said.