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The world moved closer towards a swine flu pandemic last night after the World Health Organisation (WHO) raised the threat level and the first cases emerged in Britain.

The WHO raised its assessment one notch to a phase four alert. The move came a few hours after Britain confirmed that two people being treated in a Scottish hospital were carrying the new strain of swine influenza virus that is spreading across the world from Mexico.

Nicola Sturgeon, the Scottish Health Secretary, said that the two, understood to be a young couple from Falkirk, were recovering well after being treated in isolation at Monklands Hospital in Airdrie, Lanarkshire. A further seven people who had come into contact with them had developed mild flu symptoms.

There are at least 24 other suspected cases of swine flu infection being investigated in the UK since the weekend, including a Canadian woman, treated at a hospital in Sale, Greater Manchester, but it is thought that she is unlikely to be carrying the infection.

Swine flu has been linked to up to 152 deaths, all of people aged between 20 and 50, in Mexico, where another 925 people were being tested for the virus.

As two dozen more suspected cases were being checked urgently across Europe and elsewhere, the Foreign and Commonwealth Office advised against all but essential travel to Mexico. “British nationals resident in or visiting Mexico may wish to consider whether they should remain in Mexico at this time,” it added.

Health checks for all passengers using British air and sea ports are due to be stepped up today. Alan Johnson, the Health Secretary, said that despite the trepidation, there was no need to close borders or limit travel abroad.

Mr Johnson noted that no cases of serious illnesses had been reported outside Mexico and reiterated that Britain was among the best-prepared countries for a possible pandemic, with a stockpile of 33.5 million doses of antiviral treatments.

The US Government announced that the number of confirmed cases in the US had doubled to 40 since Sunday, with the 20 latest coming from a New York City school whose students spent the Easter holidays in Mexico. There have also been confirmed cases in Canada and a single case in Spain, where a 23-year-old man became the first case to be confirmed in Europe yesterday afternoon.

There is no confirmed evidence of human-to-human transmission outside Mexico but there were further reports of suspected cases in Ireland, New Zealand, France, Israel, Scandinavia, Canada and Belgium.

The phase three level, first set in 2005 to highlight the risk of the H5N1 avian flu strain, had not been surpassed until yesterday’s decision, taken at an emergency meeting in Geneva that had been brought forward from today. It is estimated that a pandemic, were it to occur on the scale of events in 1918, 1957 and 1968, could claim millions of lives.

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