The number of flu cases in England and Wales is heading for a nine-year high, figures suggest.
GP surveillance data has shown a 75% rise in cases in a week and a 73% increase on last year's figures.
It also appears that the number of cases has started to rise among elderly patients and not just in healthy younger groups as previously reported.
Experts are urging anyone over the age of 65 or with a chronic condition to make sure they get a flu vaccine.
Flu has been rising sharply in recent weeks, prompting GPs to prescribe antiviral drugs in high-risk patients who fall ill.
Professor John Oxford suggests people stop kissing and hugging, and cough into their elbow
The latest figures show 69 GP consultations for flu-like illness per 100,000 of the population up from 39.5 per 100,000 on 14 December.
Public health officials said 200 cases per 100,000 would signal an epidemic.
In Northern Ireland there has also been a sharp rise in reports of flu and levels are now higher than in recent years but in Scotland cases seem relatively low.
Experts believe the unusually cold weather may have contributed to the surge in flu cases.
Dr Douglas Fleming, director of the Royal College of GPs Research Unit in Birmingham told the BBC the increased numbers were "significant".
"In the past ten years, the only substantial outbreak was in 1999/2000.
"I think we could be looking at something that approaches that this year."
Professor John Oxford, an expert in virology at Queen Mary's School of Medicine and Dentistry in London said he sincerely hoped the predictions were wrong as in 1999/2000, excess winter deaths hit 20,000, largely as a result of flu.
"It was not a very good year at all and it shows that anyone in an at-risk group should get themselves vaccinated.
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"The good news is there's lots of vaccine around."
The last official epidemic of flu was in 1989/1990.
"That one caught everyone a bit off guard but there's been a big push on flu vaccination since then," he added.
Professor David Salisbury, director of immunisation at the Department of Health said vaccine take up this year had been good but anyone eligible who had not yet been immunised should take up the opportunity.
"We have had a very unusual run of winters with almost no flu, so we should not be surprised that here is a winter with more flu.
"It is very difficult to predict what makes the change winter to winter."
FLU CONSULTATION RATES BY AGE
0-4 years - 57 per 100,000
5-14 years - 50 per 100,000
15-44 years - 80 per 100,000
45-64 years - 76 per 100,000
Over 65 years - 45 per 100,000
Professor Steve Field, chair of the Royal College of GPs, said he had been doing extra sessions in the surgery to take some pressure off.
"We have had people of all ages coming in and we've seen quite a lot of patients with quite severe flu-like symptoms."
He added: "It does seem to be affecting the elderly as well as the middle-aged."
It is hard to stop the spread of flu once it takes hold, he said.
"We should really try to encourage children and adults to use handkerchiefs and wash their hands - it's about personal hygiene."
Rising rates of flu are likely to put additional pressure on the emergency services over Christmas and New Year, when GP surgeries are closed.
John Heyworth, president of the College of Emergency Medicine, said most people with flu would not need to see a health professional and would be fine within two or three days.
He said: "For the frail or elderly flu can be more serious, and they may need to come to A&E if they cannot get care in the community."
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