The prime minister told the Commons of the latest UK swine flu cases
A 12-year-old girl is among five people in the UK with swine flu after visiting Mexico, Gordon Brown has confirmed.
Four adults - one from Redditch, Worcs, one in London, and a couple from near Falkirk - also have the virus.
The prime minister said the five were responding well to treatment. The girl's school in Devon has shut, with pupils offered anti-viral drugs.
The World Health Organization has raised its alert to level five - one short of a full-blown pandemic.
The number of swine flu cases globally is rising and the US has confirmed that the death of a 23-month-old Mexican child in Texas was the first fatality outside Mexico.
The BBC's Alex Bushill in Paignton, Devon
No school expected to be at the centre of a national media storm, but for the pupils of Paignton Community and Sports College in south Devon, there were to be even more shocks and surprises.
A little after 1300 BST this afternoon, the 2,000 pupils were told that one of their friends had contracted swine flu from a recent trip to Mexico. They did not know it, but already satellite vans and cameramen were gathering outside the school gate and the prime minister had announced on the floor of the Commons that a 12-year-old girl at the school was ill with the virus.
Although she is now responding well to drugs, there is a great deal of anger amongst parents and pupils alike that they were not told in advance.
In amongst the tears, parents came to collect their children as the school was closed. Reeling not just from the shock that swine flu had arrived on their doorstep, but in the manner that they had heard the worrying news.
The Department of Health said the other confirmed adult cases involved a 41-year-old woman from Redditch, Worcs, and a 22-year-old man from south London.
At a press conference, Health Secretary Alan Johnson said that the 12-year-old in Devon had been on the same flight from Mexico as the Askhams.
The closure of her school, Paignton Community College, will be reviewed after seven days.
Mr Johnson said that all UK cases of swine flu so far were linked directly to Mexico and said the government would increase stockpiles of anti-viral drugs to provide for 50 million people.
He added that a print, TV and radio advertising campaign would be launched on Thursday which would "warn the public about swine flu and remind people to cover their noses and mouths with tissues and then throw the tissue away".
Later in the House of Commons, he said supplies of antibiotics would be increased to deal with any complications arising as a result of flu infections.
"The threat we face is serious but we have never been as prepared for a pandemic as we are now," he told MPs.
He also ruled out any need for slaughtering pigs in order to combat the spread of disease.
Chief Medical Officer for England, Sir Liam Donaldson, said masks had been ordered for NHS staff - but warned that they had "very little value" to members of the public and gave a "false sense of security".
The Conservatives have hit out at the government because a national pandemic flu helpline set out in the government plans is not expected to be operational before the autumn.
From Thursday however, the public will be able to call a telephone number for recorded information about the virus.
Mr Cameron said that without a flu line there was a danger that NHS Direct could be "swamped" with calls.
Welsh Health Minister Edwina Hart told the National Assembly for Wales that as well as the five confirmed cases, a further 76 patients were being tested across the UK, 20 of whom were in Wales.
No cases have yet been reported in Northern Ireland.
The Health Protection Agency (HPA) said it was not releasing details about any suspected cases in England until swine flu was confirmed.
SYMPTOMS - WHAT TO DO
Swine flu symptoms are similar to those produced by ordinary seasonal flu - fever, cough, sore throat, aches, chills and fatigue
If you have flu symptoms and recently visited affected areas of Mexico, seek medical advice
If you suspect you are infected, stay at home and take advice by telephone initially, in order to minimise the risk of infection
Swine flu information line available from Thursday 0800 1513513
The travel agents First Choice confirmed that Mr and Mrs Askham, as well as the 12-year-old girl, all arrived into Birmingham International Airport on a Thompson flight from Cancun on Tuesday 21 April.
In a statement, First Choice said its primary concern was the welfare of its customers, adding that it was co-operating with the authorities.
Torbay NHS Care Trust said the 12-year-old in Devon was in isolation at home and had "already responded well to treatment".
Health officials in the West Midlands said the infected woman in Redditch was being treated at home with anti-viral medication, and was responding well to treatment.
The World Health Organization (WHO) has raised the alert over the spread of swine flu to level five - one short of a full-blown pandemic.
A phase five alert means human-to-human transmission in at least two countries.
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