Medical professionals have said jabs are the "best defence" against the highly- infectious disease which has hit the UK earlier than usual this year.
More people over the age of 65 have been vaccinated this year compared with 2018, but less than one third of two and three-year-olds have had their jab.
The number of patients booking doctor's appointments with flu-like symptoms has increased by 24 per cent in the last week, according to the latest data from Public Health England.
NHS National medical director Stephen Powis urged the 25 million Britons eligible for the free vaccination to "get your jab now".
Professor Powis said: "We know that children are 'super spreaders' of flu, particularly around the holiday season when they're more likely to see elderly relatives.
"So our message is simple: the flu season is here, get your jab now.
"It might be the difference between a Christmas to remember, and one to forget."
This winter, Public Health England (PHE) refreshed its Catch It, Bin It, Kill It campaign aiming to prevent the spread of the highly infectious disease with hygiene advice.
PHE medical director Professor Yvonne Doyle also urged parents with children aged two and three to get them vaccinated "as soon as possible".
She said: "The health service in England has prepared for its largest ever flu protection drive to help keep people well and ease pressure on urgent care services over the colder months."
Chief medical officer Chris Whitty said increasing numbers of people catching the infection puts pressure on hospital services, particularly as the disease worsens health conditions common among older people.
Professor Whitty said: "The winter flu season has started early in the UK and it is important that everyone who is eligible gets the flu jab from their GP or pharmacist.
"For older people and those with underlying health conditions, getting flu is particularly bad news because it can lead to really serious conditions like pneumonia and bronchitis, which can mean a lengthy stay in hospital.
"The vaccine is the best defence against flu whilst practising sensible hygiene can reduce the spread of flu."
People aged over 65, young children, pregnant women and those who have underlying medical conditions are particularly encouraged to book their flu jab, and get a nasal spray from a pharmacy.