Keiji Fukuda, the WHO’s assistant director-general, said that while the hike was a “significant step towards pandemic influenza, it is also a phase which says we are not there yet.” He added that “a pandemic is not considered inevitable at this time”.

Anyone arriving into Britain or checking in for flights and crossings today will be asked if they are suffering flu or showing flu-like symptoms. They could be sent to a local hospital to have samples taken from nose and throat swabs, the Health Protection Agency said. President Obama, speaking about outbreaks in the United States, said that it was “obviously a cause for concern”. But, he added, in his first comments on the issue, “it is not a cause for alarm”.

Michael Bloomberg, the Mayor of New York, where there were 28 confirmed cases, said that there were concerns that many more students at the Saint Francis Prep High School in Queens could become infected. Six of the students with confirmed swine flu travelled to Mexico.

The acting head of the Centres for Disease Control and Prevention warned that the US should be prepared for the problem to become more severe and that there could be deaths. WHO teams are still trying to find out why illness caused by the strain in Mexico appears to be more deadly than that in the US.

Andorra Vassiliou, the EU Health Commissioner, said that people “should avoid travelling to Mexico or the United States of America unless it is very urgent for them”. Official travel advice from the Foreign and Commonwealth Office is less restrictive.

The British Government and the Scottish Executive said that they had significant stockpiles of two anti-viral drugs — Tamiflu and Relenza — both of which have been effective in treating the H1N1 strain. The Government has agreements with manufacturers should a vaccine be developed, although Mr Johnson acknowledged it would be some time before that could happen as the virus was not yet sufficiently understood by scientists.

SIX LEVELS OF ALERT

Phase one A virus circulates among animals but has not infected humans

Phase two Animal flu virus is known to have infected humans and is considered a potential pandemic threat

Phase three A small outbreak of an animal or human-animal virus in humans. The level of human-to-human transmission is not sufficient enough to sustain community-level outbreaks.

Phase four A significant increase in the risk of a pandemic. Affected countries should impose trade and travel restrictions and consider deploying a vaccine, although one is not available in this case. This phase still offers the possibility of isolating affected areas

Phase five Pandemic is “imminent”. Affected countries are required to advise people with respiratory illnesses to stay home and schools or businesses should prepare to suspend classes or meetings

Phase six Sustained transmission of a virus between humans in several countries. Contingency plans for health systems at all levels should be implemented

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