Flu cases almost tripled last week in England and Wales as this year's outbreak strengthened its grip.

The number of cases rose from 32.8 to 87.1 per 100,000 of the population, well above last winter's peak. The rates are highest among school children, according to the Royal College of GPs surveillance unit in Birmingham, which collects figures from 69 surgeries across the country.

Cases are concentrated in the south and central regions with lower rates in the north. Most cases are H1N1 swine flu but two other strains – H3N2 and Type B – are also circulating. The Department of Health released figures on Tuesday showing there were 302 people in intensive care with flu, 70 per cent above last winter's peak, and 17 had died.

The high numbers in intensive care puzzled specialists because there appeared to be little flu spreading in the community. Now its spread is accelerating, according to the RCGP figures.