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.