There was a sharp rise in hospital admissions in England for confirmed flu cases in the last week of December, Public Health England figures show.
There were 114 admissions to intensive care with confirmed flu and a further 421 people admitted to general wards – up from 61 and 66 the week before.
Prof Paul Cosford, medical director at PHE, said the rises were "significant" but not unexpected.
And he said it was still not too late to be vaccinated.
Adults aged over 65, pregnant women and those with underlying health conditions are advised to get a free flu jab.
A flu nasal spray is available free to children aged two to eight, who are thought to be the main spreaders of flu.
NHS bosses had previously warned of a bad flu season in the UK after Australia experienced its worst flu season for a number of years during their winter.
But Prof Cosford said it was too early to say exactly how severe the flu season would be this year.
"Our data shows that more people are visiting GPs with flu symptoms and we are seeing more people admitted to hospitals with the flu.
"This is contributing to the pressure that we see the NHS under."
He said there were simple steps people could take to help prevent flu spreading.
"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."
How do I know if I have flu?
Symptoms can include:
- a sudden fever – a temperature of 38C or above
- aching body
- feeling tired or exhausted
- dry, chesty cough
- sore throat
- difficulty sleeping
- loss of appetite
- diarrhoea or tummy pain
- nausea and being sick
The PHE figures show there was also a rise in the number of reported flu-like cases seen by GPs.
Because of bank holidays, GP surgeries were open for three days in the last week of December, but in this period the consultation rate was 21 per 100,000 in England, compared to 18.9 per 100,000 the previous week.
This is above the baseline level of 13.1 per 100,000 but still classed as "low".
In the last severe flu outbreak in 2010-11 the consultation rate rose to more than 120 per 100,000.
Increases in GP consultation rates for flu were also seen across the UK, with rates in Scotland increasing the most.
Data from Health Protection Scotland shows that the number of people suffering from flu in Scotland has more than doubled compared to the same time last year.
Early tests suggest that just over half of the circulating strains of flu match those in the 2017/18 vaccine, the Scottish government said.