BRITONS were warned to avoid all but essential travel to Mexico today as world health officials said the deadly swine flu virus can no longer be contained.
The UK braced itself for further cases as officials issued new travel advice after the World Health Organisation (WHO) raised its alert over the outbreak to level four, signifying a "significant increase in risk of a pandemic".
The first two British cases were confirmed yesterday and more suspected infections emerged last night.
The Foreign Office said: "We are now advising against all but essential travel to Mexico."
It added that British nationals "resident in or visiting Mexico may wish to consider whether they should remain in Mexico at this time".
The two confirmed patients, Iain and Dawn Askham, of Polmont, near Falkirk, had reportedly been on honeymoon in Mexico and were being treated today in isolation at Monklands Hospital in Airdrie, Lanarkshire.
Scottish Health Secretary Nicola Sturgeon said yesterday they had tested positive and seven of 22 people who had been in contact with them were showing "mild symptoms".
UK holiday company Thomson said that it was cancelling two outbound flights from the UK to the Mexican resort of Cancun today and repatriating holidaymakers already in Mexico.
The flights to Cancun had been due to leave from Manchester at 9.30am and from Gatwick at 9.15am.
Thomson said it was now making arrangements to get clients home from Mexico and that repatriation flights were expected to start today.
The company said: "While we do appreciate that the new news may be a great disappointment to customers, we hope they will also understand that their health and safety is of paramount importance to us."
It added that flights due to operate to Mexico from tomorrow onwards were under discussion and decisions would be made later today.
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 the United States, Canada and Spain.
Suspected cases from New Zealand to Israel were raising concern that the new virus was spreading rapidly.
It was also confirmed that four people in the Republic of Ireland were being tested.
The Health Service Executive (HSE) in Dublin said the results of those tests were expected this morning.
WHO’s assistant director-general Keiji Fukuda said "at this time containment is not a feasible option" as the virus has already spread to several other countries.
He added: "At this time instituting travel bans would really not be very effective as the virus has already spread to several other countries."
Commenting on the raising of the WHO’s alert level to four - two lower than the maximum alert level of six - he said: "What this can really be interpreted as is a significant step towards pandemic influenza. But also, it is a phase that says we are not there yet.
"In other words, at this time we think we have taken a step in that direction, but a pandemic is not considered inevitable."
Professor Sir Liam Donaldson, the Government’s Chief Medical Advisor, said: "The WHO has changed the alert to stage four. This means there are small clusters of outbreaks with limited person-to-person transmission.
"From some affected areas, it appears that early doses of antivirals such as Tamiflu are helping people to recover."
He said there were enough antivirals to treat "half the population" if they became ill.
He added: "We have been preparing for the possibility for a number of years and are among the most prepared countries in the world."
Ms Sturgeon told a press conference in Edinburgh last night: "I would reiterate that the threat to the public remains low and that the precautionary actions we have taken over the last two days have been important in allowing us to respond appropriately and give us the best prospect of disrupting the spread of the virus."
In the US, the number of cases rose to 50, the result of further testing at a New York City school, although none was fatal. Other US cases have been reported in Ohio, Kansas, Texas and California. Six cases were confirmed in Canada and a further one in Spain.
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.
A passenger on an incoming flight from Mexico was treated by medics at Manchester airport this morning, the Health Protection Agency said.
The passenger was on flight TCX234 from Cancun. It landed at 7.33am.
A HPA spokesman said: "We can confirm that paramedics supported by airport health officials and the Health Protection Agency treated a passenger who became ill during the flight from Mexico to Manchester airport earlier today.
"Tests are being undertaken in an attempt to establish the cause of illness."
Sir Liam, speaking on GMTV, said: ``The WHO has raised its alert level, as we all know. That does not mean that a pandemic is inevitable but we are very concerned. It does seem as if this disease is a new strain of flu, it is one that is spreading quite rapidly from person to person.
"So our contingency plans for combating a pandemic in this country have been put into place and we are doing all the things that really we thought we would need to do when this pandemic arrived."
Dr Maureen Baker, the honorary secretary of the Royal College of GPs, said family doctors should reassure patients that they can expect to recover from flu within a week - even if it is swine flu.
Dr Baker told the BBC Radio 4 Today programme: "GPs should be saying to people not to panic, that we are well prepared and that most people who get ill with flu will have a nasty bout of flu and get better within a week, even if it is swine flu.
Ms Sturgeon told BBC Radio Scotland today that the seven who had developed symptoms of the illness had been in ``close contact'' with the infected pair, including workmates.
"We remain very vigilant," she said today.
"At the moment we have no evidence that the virus is in the wider community, although people will understand I obviously can’t rule that out.
"I would say in terms of reassurance that the one thing everyone in the international community is taking encouragement from is that, outside of Mexico, everybody contracting this virus is displaying very mild symptoms."
She added that the two infected individuals were responding well to anti-viral drugs.
Sir Liam told BBC Breakfast: "We have been planning for this eventuality for over five years now. We knew that at some point a new strain of flu would emerge and that it had the potential to produce a pandemic so we have had the health service preparing and rehearsing for some time.
"One of the most important measures is to be able to get anti-viral drugs to people who are affected as quickly as possible.
"They won’t cure the flu but they will reduce the severity of the attack and indeed possibly prevent it spreading to other people that the person is in contact with."
Asked what changes level 4 meant, he said: "At this point, it does not affect our plans a great deal because we are still getting small numbers of cases in the UK and we are not getting onward transmission from the people affected.
"When we start to see that, then our plans will enter a different phase. We will be giving people much more information about what they need to do. We will be giving more information to doctors and will be arranging for the distribution of anti-viral drugs from our stock pile to a local level."
Passengers disembarking from flight TCX234 at Manchester Airport said the passenger who was taken ill was a young woman.
Pauline Jones, in her 60s, from West Kirby, Wirral, said she was quite alarmed.
She said: "I think the fatalities are all in Mexico City. You can’t believe that anything could happen to you."
She and her husband had a two-week holiday in Cancun and she said she would return to Mexico.
Another woman said: "We never knew anything about it until just before we left."
Ian Inglis, 31, from Sunderland, said: "Everything was fine over there, it was a good holiday and I would go back."
Colin Pete, 50, of Southport, said: "We found out (about swine flu) three days ago. It was OK, because I read on the BBC website that it was all in Mexico City. It was as if it wasn’t there (in Cancun)."
Health Secretary Alan Johnson is to chair a further meeting of the Government's Cobra emergencies committee this morning, the Cabinet Office said.
Professor Neil Ferguson, a member of the WHO's Pandemic Influenza Taskforce which yesterday decided to raise the alert level over swine flu, said 40% of the UK population could be infected if the country is hit by 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.
The swine flu virus H1N1 was not as dangerous as the H5N1 strain known as bird flu, and it was possible that the rate of deaths in Mexico was lower than in the last major pandemic, the so-called Hong Kong flu outbreak of 1968, which killed one in 1,000 of those who were infected.
Prof Ferguson told the BBC Radio 4 Today programme: "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.
"We could get substantial numbers infected in the next few weeks but, if I was to be a betting man, I would say it would be a slightly longer period of time, just because we are moving into the summer months."
Prof Ferguson said the 152 deaths in Mexico probably make up a relatively small proportion of the total number infected, who might run into tens or hundreds of thousands.
"So-called bird flu - H5N1 - was a much more dangerous virus," he said. "We are not in the same ballpark. But we can’t at the moment answer the question ’Is it comparable to 1918 Spanish flu, which killed a lot of people, or is it much more like Hong Kong flu?’."
The virulent Spanish flu of 1918 spread to all parts of the world, infecting as many as a third of the global population. Around 40 million people are thought to have died from the illness - 250,000 of them in the UK - though some estimates range much higher.