Two holidaymakers admitted to a Scottish hospital after returning from Mexico were confirmed on Monday night as the UK’s first cases of swine flu as the World Health Organisation said the virus posed a “significant risk” and increased its pandemic alert.

As the number of confirmed cases of swine flu rose in the US, Canada and Europe, ministers moved to reassure the public that the government was prepared for a pandemic, with substantial stocks of medicines and extensive plans for dealing with an emergency.

Alan Johnson, health secretary, said the UK had “established a stockpile of enough anti-viral drugs to treat more than 33m people,” or more than half the population and Gordon Brown, prime minister, said on Tuesday said Britain had been “preparing for this kind of scenario for many years” and was “among the best prepared countries in the world”.

Professor Neil Ferguson, a member of the WHO’s pandemic influenza taskforce, said 40 per cent of the UK population could eventually be infected if the country was hit by a pandemic.

Prof Ferguson, of London’s Imperial College, said cases were likely to die down within a matter of weeks because the UK was moving out of the normal season for flu infection, but there was a risk that the disease would flare up again once the summer was over.

He told the BBC Radio 4 Today programme the 149 deaths in Mexico probably made up a relatively small proportion of the total number infected, who might run into tens or hundreds of thousands.

“So-called bird flu - H5N1 - was a much more dangerous virus,’’ he said. “We are not in the same ballpark. But we can’t at the moment answer the question ’Is it comparable to 1918 Spanish flu, which killed a lot of people, or is it much more like Hong Kong flu?’.’’

Sixteen of 25 possible cases reported in Britain were being investigated on Monday, including the two in Scotland. Last night, nine had proved negative.

The European Union’s health commissioner Androulla Vassiliou issued earlier advice, warning against travel to the US. In comments that her staff stressed were not official policy, Ms Vassiliou advised against travel to areas affected by the disease except if “very urgent”, explicitly citing first Mexico and then the US.

However, Richard Besser, acting director of the US Centers for Disease Control and Prevention, dismissed any such advice as not “warranted”, while himself advising against non-essential visits to Mexico, ahead of circulation of official US government guidance.

The spat came as specialists struggled to understand the extent of the epidemic and its potency. US President Barack Obama attempted to smooth reactions, saying of the cases in the US: “This is obviously a cause for concern and requires a heightened state of alert. But it is not a cause for alarm.”

Nicola Sturgeon, Scottish health minister, confirmed two patients were being treated in isolation in a hospital in Airdrie near Glasgow. Government officials said the pair returned from holiday last Tuesday and “presented to doctors” on Saturday.

Ms Sturgeon stressed that the two were “recovering well” and showing only “mild symptoms”. She said seven associates of the two patients had showed mild flu symptoms, but that these had not been confirmed as swine flu.

The government put in place “enhanced” health checks at airports to identify passengers arriving in the UK with symptoms, and is considering issuing face masks to health workers.

Health officials will quiz passengers, though ministers are reluctant to introduce “screening” measures such as taking swabs or people’s temperature. A swine flu alert has been issued to hospitals.

Financial markets gyrated on the first day of trading since the World Health Organisation raised the “pandemic potential” of the H1N1 swine flu virus that was first identified in Mexico. Shares in airlines and tour operators were hardest hit, amid concerns that a flu pandemic could scupper hopes of economic recovery.

Share prices of pharmaceutical companies with drugs and vaccines that could benefit from a pandemic performed well, led by Roche, which bought the licence for flu drug Tamiflu, Gilead and GlaxoSmithKline of the UK.

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