The number of flu cases across the country has doubled to two million in a week, according to data released today.
People are being encouraged to vaccinate after health officials have confirmed that influenza virus in the winter has begun.
This occurs after weeks of very little activity being reported by Public Health England (PHE).
Millions of people are suffering from symptoms such as fever, chills and body aches, and experts fear this may put pressure on the already overburdened SNS.
The virus is circulating widely throughout the UK and within a week, cases have increased 85% in England and 55% in Wales.
Meanwhile, flu reports rose 139% and 50% in Northern Ireland.
PHE publishes a weekly report showing the number of flu symptoms that were recorded in GP consultations.
Based on how often flu symptoms are reported by 100,000 patients, each country sets its own criteria for what it considers to be an epidemic.
Although none of the countries of origin has reached the point of being an epidemic, England and Wales are on the "low" threshold.
This is the second step on a five-point scale for "very high", which are epidemic levels.
Meanwhile, in Scotland and Northern Ireland, the levels are at the first limit, which is the baseline.
The Northeast, West Midlands, Northwest of England and Yorkshire and Humber have an "average intensity" of cases, which makes it the most affected.
London, the South East, East Midlands, South West and East of England are less affected and are in "low intensity".
Regional numbers are not available for Northern Ireland, Wales and Scotland.
The rates are worse since monitoring began in September and in three months, 41 people died of the disease.
The PHE data released today revealed that the UK had an average of 60 influenza diagnoses per 100,000 people last week.
And it showed that the H1N1 strain (swine flu) was the most widespread.
In the previous week there was a 76% increase, which was the last week of 2018.
PHE added that the increase in cases means that the flu is having a "moderate impact" on hospitals and intensive care units.
One-third of the flu cases this season that needed intensive treatment occurred last week – which is 205 out of a total of 666.
PHE is advising who is eligible to get the flu jab if it has not already.
For the first time this winter, all elementary school children are now eligible for a free flu vaccine as well as children two and three years old on August 31, 2018.
The vaccine is also being offered to employees working in homes and nursing homes this winter and who have regular contact with the client.
In addition, caregivers, volunteers who provide care or provide first aid at organized public events, as well as community lifeguards, are entitled to the vaccine.
And adults who are morbidly obese – with a body mass index of 40 or more – can also have a jab for free.
Those who are not eligible for a free NHS vaccine – like healthy adults – can pay for one at their local pharmacy between £ 8 and £ 10.
To improve effectiveness, different influenza vaccines recommended for different age groups are being introduced this year.
There are four different types in total – one for children (nasal spray), two for adults under 65 and one for people 65 and older.
If you think you have the flu, check out the symptom checker at www.nhs.uk/conditions/flu.