Children under five are now the worst affected by this year’s flu outbreak with cases at their highest level in winter since an epidemic a decade ago.
Official figures showed that children under the age of five had the highest rates of flu of any age group, with 184 cases per 100,000.
Young children have overtaken those aged five to 14 after rates dropped among older children because of the school holidays.
The figures will increase the controversy about whether young children should have been offered the seasonal flu vaccine.
Last winter, all children under five were offered the swine flu vaccine on the NHS, but a government advisory committee did not recommend that this should be repeated this winter.
Yesterday’s figures from the Royal College of GPs showed that the sharpest increase in cases had been in adults aged 45 to 64, with the number of consultations almost doubling to 108 per 100,000 of the population.
Overall, cases of flu increased by almost half in the week to Boxing Day, with 124 people reporting symptoms per 100,000.
The last time seasonal flu cases were this high was during the 1999-2000 flu epidemic, when an estimated 22,000 people died. A flu epidemic is declared when the number of GP consultations about symptoms reaches 200 per 100,000.
Rates were also higher than last week’s levels during the 2008-9 swine flu pandemic, but most of those cases occurred during the summer.
Officials are releasing batches of the antiviral drug Tamiflu left over from the pandemic to ease shortages of the medicine.
Officials said flu rates remained within expected levels for this time of year but cases could surge after the new year as children go back to school.
NHS Direct had its busiest day of the year on Monday after receiving more than 30,000 calls, almost one in seven of which were about symptoms of colds and flu.
Up to one in four intensive care beds in some parts of the country is being used for flu patients as NHS services come under increasing pressure.
Norovirus cases also peak at this time of year, leading to ward closures and staff shortages as patients and nurses fall ill.
A political row has broken out after it emerged that independent experts were considering whether to advise the routine vaccination of under fives against seasonal flu.
The current seasonal flu vaccine protects against the H1N1 swine flu virus along with two other strains of flu, whereas the pandemic vaccine, which was offered to all under-fives, only protected against H1N1.
Pregnant women have been offered the seasonal vaccine this year for the first time because they are at greater risk of complications if they contract swine flu. But under-fives, who were also hit hard by last year’s pandemic, were not included in the groups offered the seasonal jab.
The weekly report from the Royal College of GPs said: “Compared with last week’s equivalent rates the incidence of influenza-like illness has increased by about 50 per cent.
“The increase was evident in all age groups except schoolchildren (for obvious holiday reasons). There has been a substantial increase in age groups 45-64. Increased incidence of other respiratory illnesses are small.”
Flu consultations doubled in the North to reach 118 per 100,000 and there were smaller increases elsewhere. Flu rates remain highest in the South at 136 per 100,000 people.
There have been 27 deaths linked to flu so far this winter, according to figures from the Department of Health.
Meanwhile, an NHS pressure group has warned that the outbreak could expose a “desperate” shortage in intensive therapy capacity.
Health Emergency claimed there was a lack of capacity and specialist nurses that they said could put lives at risk if the flu outbreak gathered momentum.
Geoff Martin, the chairman of Health Emergency, said: “We are getting reports of intensive care units in London where up to a quarter of the beds are filled with swine flu cases and the crisis is getting worse by the day.
“Cuts in recent years to bed and staff numbers have left the NHS dangerously exposed and there is no doubt that many [intensive care units] will soon have to close to new admissions.”
He said the Government’s “inaction on this growing crisis is outrageous” and called for the money being spent on reorganisation to be invested in intensive care.
The Department of Health said the NHS was coping well nationally and around one in seven intensive care beds was being used for flu patients. A spokesman added: “These figures are in keeping with what we would expect during a winter flu season.
“But everyone can do their bit to help keep well – simple measures like washing your hands help stop flu spreading.
“The Chief Medical Officer has issued clear advice to get the seasonal flu jab if you are in a vulnerable group, particularly pregnant women and people with underlying health condition, as well as those aged 65 and over.”