Experts say the swine flu virus may have hit thousands of children without them even knowing it - with as many as a third of school-age youngsters contracting it in parts of England.

But according to a breakdown from the Health Protection Agency, fewer than one in 10 have actually been ill with it.

Director of the HPA’s centre for infections Professor Maria Zambon said: “When we actually put together all the different pieces of information it suggests that up to a third of children in particular regions of England were infected in the first wave.”

Across the UK the figure fell to an average of approximately 15 per cent to 20 per cent of under-15s, she said.

The HPA is currently reviewing the number of people across the UK who may have had the virus.

Previous estimates have been that a million people have been affected.

Professor John Watson, head of the HPA’s respiratory diseases department, said half or more of all people with swine flu would have had no symptoms.

However, he said the rates of complications in some groups, including the under-fives, were high.

The HPA said it was unclear what course the current flu pandemic would take or if there would be a second spike in swine flu cases.

Prof Zambon admitted the pandemic had not been as serious so far as the estimates set out in planning documents.