The number of people admitted to intensive care with flu has risen by 65%, the latest figures show.
Public Health England (PHE) said there had also been a 78% increase in the GP consultation rate with flu-like illness, and a 50% increase in the rate of hospital admissions for flu cases in the first week of the year.
The main strains circulating continue to be flu A(H3N2), known as Aussie flu, A(H1N1), known as swine flu, and Flu B.
There have been 48 flu-related deaths in England so far this winter.
Professor Paul Cosford, medical director at PHE, said: "Our data shows that more people are visiting GPs with flu symptoms and we are seeing more people admitted to hospital with flu.
"We are currently seeing a mix of flu types, including the A(H3N2) strain that circulated last winter in the UK and then in Australia. The A(H3N2) strain particularly affects older, more vulnerable age groups.
"People suffering with flu-like symptoms should catch coughs or sneezes in tissues and bin them immediately, wash their hands regularly with soap and warm water and frequently clean regularly-used surfaces to stop the spread of flu. Avoid having unnecessary contact with other people if you or they have symptoms of flu."
Seasonal flu and other diseases that normally increase during winter, such as norovirus, put extra pressure on the NHS every year.
Prof Cosford urged those who are eligible to take up their offer of the flu vaccine.
PHE said 71.3% of adults over 65, 46.9% of adults with a long-term health condition, 45.5% of pregnant women, 40.8% of three-year-olds and 42% of two-year-olds have received the vaccine this winter.