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Thousands of British tourists were preparing 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.

The first Britons to have contracted the disease were named this morning as Iain and Dawn Askham of Polmont, near Falkirk in Scotland, as fresh cases were reported as far afield as Israel and New Zealand.

The couple fell sick after returning from their honeymoon in the resort of Cancun but are said to be recovering well after being treated with anti-viral drugs in an isolation ward at Monklands Hospital in Airdrie, Lanarkshire. Gordon Brown said this morning that one of them may be well enough to leave hospital today.

The disease has killed 152 people in Mexico since it was identified there on April 13 but British tourists have so far shown characteristic defiance and were continuing to fly off on their holidays to the resorts of the Yucatan Peninsula.

Last night, however, the WHO upped the threat level posed by the virus to level four on a scale of six and the Foreign and Commonwealth Office formally advised against all non-essential travel to Mexico.

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 are to be stepped up today. Anyone arriving in 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 (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.

"We have not seen any contact with pigs or with other sources of transmission, so we have to come to the conclusion that there is human-to human-transmission," Gregory Hartl, the WHO spokesman, told a press conference in Geneva.

"In the UK at least one of the two cases appears to have been imported, as apparently this was a couple on honeymoon in Mexico, but whether they both got infected there or did one infect the other [in Britain], we do not know.

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