Cases of flu rose by almost 50% last week, according to figures released by doctors.
The Royal College of General Practitioners (RCGP) reported that incidences of flu in England and Wales reached 124 per 100,000 of the population in the week ending December 26. There were 86 per 100,000 cases in the previous week.
There has been a general increase across all age ranges apart from children, with the middle-aged being particularly badly hit.
An RCGP spokesman said: "Compared with last week's equivalent rates the incidence of ILI (influenza-like illness) has increased by about 50%. The increase was evident in all age groups except schoolchildren. There has been a substantial increase in age groups 45-64. Increased incidence of other respiratory illnesses are small."
Experts believe the number of schoolchildren catching the flu could change when they return to school following the Christmas break.
The increase still falls short of epidemic levels, which experts define as 200 cases per 100,000.
A Department of Health spokesman said: "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 an underlying health condition, as well as those aged 65 and over."
The figures were released as an NHS pressure group warned that the flu outbreak could expose a "desperate" shortage in intensive therapy capacity. Health Emergency claimed there was currently a lack of ITU capacity and specialist nurses which they said could put lives at risk if the flu outbreak gathered momentum in the New Year.
Geoff Martin, 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 ITUs will soon have to close to new admissions, putting hundreds of lives at risk."
But the Department of Health insisted the NHS was coping well with the flu outbreak. A spokesman said: "Our latest data shows that the number of people with confirmed or suspected flu in critical care beds is 460. This represents less than one in seven of the total critical care beds available. The NHS is coping very well and only a small percentage of the intensive care capacity is being taken up with patients with flu."