Family doctors are seeing 69 people with flu symptoms for every 100,000 members of the population in England and Wales, according to figures released this week.
The rate has shot up almost three-quarters from the 40 people per 100,000 that doctors were seeing the previous week.
The rise was recorded by the Birmingham Research Unit of the RGCP.
Unit director Douglas Fleming said rates of infection had increased in all regions and across all age groups.
"The recent increase is suggestive of an influenza outbreak greater than any seen in the last eight years," he said.
The news came a day after health officials raised concerns that a rise in flu cases could put further strain on hospitals and A&E departments over Christmas.
An influx of related to the cold weather earlier this month has prompted experts to warn that hospitals are under "intense pressure".
They are concerned the situation will get worse this week when many doctors' surgeries will shut for four days.
The Department of Health's director of immunisation, Professor David M Salisbury, said: "Flu has started earlier this year than last year and today's figures show an increase in the number of people consulting their GPs with flu-like illnesses.
"We have had very little flu over the last few years and this year may be like 1999/2000 when figures were quite high.
"The best protection against flu is to have the flu jab.
"If you are over 65 or in one of the risk groups and have not had a jab this year, it's not too late.
"Contact your surgery to check if they still have supplies and make an appointment - it won't be too late, even after Christmas."
He added: "There are simple steps that everyone can take to help prevent catching colds and flu.
"Always use a tissue to catch your sneezes, throw away used tissues where germs can linger and regularly wash your hands.
"If you think you have flu, stay at home, drink plenty of fluids and take medicines like aspirin or paracetamol for your symptoms."
