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The World Health Organisation issued a historical note of warning over swine flu today when a top official pointed out that the Spanish flu pandemic of 1918 - which killed at least 50 million people - also started off as an apparently mild virus.

Dr Keiji Fukuda, assistant director-general at the UN health agency, said that there was still no good explanation as to why so many cases of the virus have proved fatal in Mexico yet cases in other countries have been easily treated with anti-viral drugs.

Referring to the Spanish pandemic, he warned that the outbreak was unpredictable. “I think we have to be mindful and respectful of the fact that influenza moves in ways we cannot predict,” he said, in a telephone briefing from Geneva.

Mexican officials say that around 150 people have died of the new virus strain since the outbreak began in February. The WHO cautions, however, that only seven of those deaths have been confirmed in the laboratory as being caused by swine flu.

So far there are only two confirmed cases in the UK - a Scottish couple who developed flu-like symptoms after returning last week from their honeymoon in the beach resort of Cancun last week. Iain and Dawn Askham of Polmont, near Falkirk, appear to be responding well to treatment in an isolation ward at Monklands Hospital in Airdrie, Lanarkshire.

Nicola Sturgeon, the Scottish Health Secretary, said today that nine of 22 people with whom the Askhams came into close contact after their return have developed mild symptoms but it would not be known until tomorrow whether any of them had picked up the virus. Another 14 people across Scotland are being tested for the new H1N1 strain, all of them recently returned from either Mexico or areas of the United States hit by outbreaks.

Thousands of British tourists prepared to cut short their Mexican beach holidays today after the World Health Organisation upped the threat level posed by swine flu to just one step short of a pandemic and the Foreign and Commonwealth Office formally advised against all non-essential travel to the country.

The FCO decision immediately prompted major tour operators to cancel all new holidays to Mexico, although they have yet to decide whether to start repatriating the 10,000 Britons currently there.

Health checks for all passengers using British airports and sea ports have been stepped up today. Anyone arriving in Britain or checking in for flights and crossings were asked if they were 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 (HPA) said.

The WHO's decision recognises that the disease is spreading and transmitting itself in localised areas but had not yet taken sufficient hold to be described as a pandemic.

The UN health agency said today that it was clear that human-to-human transmission of the virus – one of the key definitions in declaring a pandemic – was happening in Mexico and the United States but it was not clear whether it had yet occurred in Britain or any of the other countries with multiple confirmed cases.

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