The deadly swine flu virus could infect up to 40% of the UK population in the next six months if the outbreak becomes a pandemic, world health officials have warned.

Professor Neil Ferguson, a member of the World Health Organisation (WHO) taskforce which raised its alert over the virus to level four, said four in 10 people could be infected if the country was hit by a pandemic.

But Prime Minister Gordon Brown insisted the UK was "among the best prepared countries in the world" and the Government was taking "all the urgent action that is necessary" to help prevent the virus spreading.

Earlier, Britons were warned to avoid all but essential travel to Mexico as holiday companies suspended their operations to the country.

Tour operator Thomson, which cancelled two flights due to leave the UK for the Mexican resort of Cancun, said it was bringing home all Thomson and First Choice holidaymakers already in Mexico. And all Thomas Cook and Airtours holidays to Cancun scheduled for the next seven days were cancelled.

The WHO said the deadly swine flu virus can no longer be contained and raised its alert to two lower than the maximum of six, signifying a "significant increase in risk of a pandemic".

Prof Ferguson, of Imperial College, London, 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 may flare up again once the summer was over.

He said: "We don't really know what size epidemic we will get over the next couple of months. It is almost certain that, even if it does fade away in the next few weeks - which it might - we will get a seasonal epidemic in the autumn.

"We might expect up to 30%-40% of the population to become ill in the next six months if this truly turns into a pandemic."

More suspected infections have emerged since the first two British cases were confirmed on Monday. The two confirmed patients, Iain and Dawn Askham, of Polmont near Falkirk, had been on honeymoon in Mexico and remain on an isolation ward at Monklands Hospital in Airdrie, Lanarkshire.