The UK has been put on alert as the global health watchdog warned countries to look out for unusual flu cases after a deadly outbreak of swine influenza.
More than 80 people have died of pneumonia in Mexico after contracting a flu-like virus and many others - including children in a New York school - have fallen ill in the US and Mexico.
World Health Organisation director-general Margaret Chan said the outbreak in Mexico and the United States constituted a "public health emergency of international concern".
It means nations will be expected to step up reporting and surveillance of the contagious respiratory disease, which she said had "pandemic potential".
The H1N1 strain of swine flu usually associated with pigs has spread to humans, causing symptoms including fever and fatigue.
Health authorities were monitoring the situation as tests were carried out to assess the potential of the virus to spread.
Laboratory tests showed that a man taken to hospital on Saturday with "flu-like symptoms" after landing at Heathrow Airport on a flight from Mexico City does not have swine flu.
The man, who has not been named, was a member of the cabin crew on the British Airways flight which landed at 2pm. He was taken to Northwick Park Hospital in Harrow. A hospital spokesman said: "I can confirm he does not have swine flu. All the tests have come back negative."
The UK Health Protection Agency said it was keeping a close eye on the situation involving human cases of swine influenza in case of any threat to people in this country.
An HPA spokesman said: "The HPA is monitoring this situation closely and is working with the UK government to review the current incident and any threat it poses to UK public health."
