Three new cases of swine flu were confirmed today bringing the total number of victims in the UK to 18.

The cases involved two children from London and a man from Ayrshire, Scotland.

One of the children had recently returned from the United States while the other had close contact with someone who had recently returned from Mexico.

The Health Protection Agency (HPA) said that 716 cases of possible swine flu are currently under laboratory investigation.

A Department of Health spokesman said: ''Two new cases have been confirmed in England today - both new cases in England are of school age, from the London region.

''There are now 18 confirmed cases in the UK - 14 in England and four in Scotland.

''The arrangements in place across the UK are continuing to ensure that we are well-placed to deal with this new infection.''

He said close school contacts of the child linked to a traveller returning from Mexico had been given anti viral drugs as a precaution.

The HPA has advised the school to close for seven days.

The spokesman added: ''Following a thorough risk assessment, no wider public health action is deemed to be required in relation to the patient who returned from the United States.

''No school closure is necessary as the patient did not attend school while symptomatic.''

So far, just one person in the region has been confirmed as having contracted the virus on April 30.

The woman, who had recently returned from Mexico, shared a house with two Newcastle University students living off-campus in the city.

The Scottish case was announced earlier in the day by Scottish Health Secretary Nicola Sturgeon. He is the fourth person in the Ayrshire district to contract the disease.

Health secretary Alan Johnson today acknowledged there will be more cases in the UK and warned of a second wave of infection during the autumn.

''Our evidence from all previous pandemics is that you get two phases. You get a first wave that is often very mild and then you get a much more serious wave that comes along in the autumn and the winter,'' he told BBC1's The Andrew Marr Show.

''So we have to not just deal with this outbreak now, but prepare, perhaps, for a second phase further down the line.''

Although the World Health Organisation has declared a phase 5 alert level - one step short of declaring a pandemic - Mr Johnson said that the measures taken in the UK had proved effective in limiting the spread of the disease.

''Pandemic just describes the geographic spread, it doesn't describe the severity. So far ... I think it is contained.''

Yesterday a six-year-old girl from Oxfordshire was confirmed as the youngest case diagnosed in the UK to date. There are now cases in Newcastle, Merseyside, Devon, Worcestershire, Gloucestershire, Oxfordshire, London and Central Scotland.