FOUR in ten Britons could catch the deadly swine flu virus within the next six months, leading health officials said.

Professor Neil Ferguson, a member of the World Health Organisation (WHO) taskforce which raised the alert over the outbreak to level four, said 40 per cent of people in the UK could be infected if the country was hit by a pandemic.

But Prime Minister Gordon Brown said Britain was among the best prepared countries in the world and added that the Government was taking all the urgent action that is necessary to help prevent the spread of the virus.

The Government's Cobra emergencies committee, involving both Mr Brown and Health Secretary Alan Johnson, met to discuss the issue as Britons were warned to avoid all but essential travel to Mexico.

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

Prof Ferguson, of Imperial College, London, said: "We might expect up to 30 per cent-40 per cent of the population to become ill in the next six months if this truly turns into a pandemic."

The first two British cases were confirmed on Monday and more suspected infections emerged across the UK throughout yesterday.

The two confirmed patients, Iain and Dawn Askham, of Polmont, near Falkirk, had been on honeymoon in Mexico and were being treated today in isolation at Monklands Hospital in Airdrie, Lanarkshire.

Their parents said the newlywed couple were shocked at the positive test results.

In a statement issued on behalf of their parents, Ian and Pauline Askham and Linda and Brian Colston, by NHS Lanarkshire, the families said: "We are delighted that they continue to make good progress and they remain in good spirits.

"They were both quite shocked that the result was positive for swine flu but are relieved that their symptoms have been very mild.

"They added that it was an incredibly difficult time for them and they were overwhelmed by the attention."

Later, Scottish Health Secretary Nicola Sturgeon said the couple's condition was continuing to improve.

"Their condition never gave cause for concern, nevertheless they do continue to recuperate and recover," she said.

Ms Sturgeon added that nine of 22 people in contact with them since returning home were showing mild, cold-like symptoms and were also being tested for swine flu.

As dozens of people around the UK were tested, health officials warned there would be no minute by minute update and that many of the suspected cases were likely to be negative.

Thomas Cook cancelled all Thomas Cook and Airtours holidays to Cancun for the next seven days and Thomson Holidays parent company Tui said it would not be operating any more flights to Mexico until the Foreign Office travel advice changed.

Arrangements were being made to get clients home from Mexico, Thomson said.

Shares in holiday companies also saw a second day of losses as fears over the spread of the swine flu virus kept world markets under pressure.

Holiday operator TUI Travel fell more than six per cent, Thomas Cook slipped five per cent and cruise operator Carnival, which sails to mainly Caribbean destinations, also slumped nearly six per cent in the FTSE 100 Index.

British Airways fell four per cent despite the airlines announcement that it would not be cancelling any of its four weekly flights to Mexico.

And the European Commission warned that swine flu outbreaks were likely to get worse in Europe over the next few days, but advised the public not to panic.

Health Commissioner Androulla Vasiliou said there were still only three confirmed cases in the EU - one in Spain and two in the UK - but suspected cases in Ireland, Denmark, Germany, Italy, Sweden, Greece and the Czech Republic.

So far 152 people are thought to have been killed in Mexico by the virus, which is caused when the H1N1 strain associated with pigs crosses over to the human population.

Outbreaks have also been confirmed in Scotland, the United States, Canada, Spain, New Zealand and Israel.

WHOs assistant director-general, Keiji Fukuda, said the level four alert was a significant step towards pandemic influenza.

Professor Sir Liam Donaldson, the Government's chief medical adviser, said there were enough anti-virals to treat half the population if they became ill.

"We have been preparing for the possibility for a number of years and are among the most prepared countries in the world," he said.

Dr Maureen Baker, the honorary secretary of the Royal College of GPs, said family doctors should reassure patients that they could expect to recover from flu within a week - even if it was swine flu.

In the US, the number of cases rose to 50, including 28 at a single New York City school. Other US cases have been reported in Ohio, Kansas, Texas and California. A total of 11 cases were confirmed in New Zealand, six in Canada, two in Spain and one in Israel. None of the cases outside Mexico have been fatal.

Mexican health secretary Jose Angel Cordova said the epidemic was entering an extremely dangerous phase, with the number of people infected mushrooming even as authorities were improving defences.

Have you recently returned from Mexico? Have you been tested for swine flu? Call the newsdesk on 01325-505065.