The BBC's Peter Holland reports

More than 45,000 people have been hit by aches and pains as a strain of the virus swept around the country, according to the national influenza monitoring unit.

The outbreak has led to one ambulance service moving towards a major emergency status while others have reported their busiest Christmas for years.

Doctors have said that the strain of the virus is already known and is not virulent enough to cause a nationwide epidemic.

Symptoms of 'flu include fever, cough, weakness and loss of appetite.

In the elderly and the young, and in patients with respiratory disorders, cardiovascular disease and poorly functioning immune systems it can be fatal.

Vaccinations are available, but their effectiveness depends on how closely they are matched to the strain of the disease.

The strain affecting Britain at the moment is called H3N2 Sydney 'flu after the Australian city where it was discovered.

Rise in incidence

Dr Douglas Fleming, of the Royal College of General Practitioners' 'flu monitoring unit, said: "There have been a lot of flu cases in recent days but it is at a typical level for this time of the year.

"People will be a little surprised because there wasn't a flu outbreak last year, so it will feel like a mass outbreak but the truth is it's business as usual.

"Sufferers will get all the usual symptoms from aching limbs to a streaming nose but it is extremely unlikely that it will become an epidemic according to our latest figures."

However, hospitals and ambulance crews have felt the impact of the outbreak.

The 'flu virus takes many forms

On Sunday, Staffordshire Ambulance service declared the first stage of a major emergency as crews struggled to cope with exceptionally high demand.

Emergency calls over the previous three days had been running 50 per cent more than expected for this time of the year.

On Christmas day a spokesman said they would normally expect to deal with around 200 patients but this year they dealt with around 360.

Some crews worked 18 hour shifts and managers had to appeal for help from off duty staff.

Bob Lee, a spokesman for the trust, said many of the calls appeared to be from people suffering with 'flu-type symptoms.

NHS spokesman Hugh Lamont: "Busier than a typical New Year's Eve"

Hugh Lamont, a spokesman for the NHS in north-west England, said ambulance crews in that region had handled more than 2,000 calls over the last 24 hours.

He said on Monday: "Yesterday was busier than a typical New Year's Eve.

"There's a lot of pressure on A&E departments , and people are having to wait longer than we would like to get into hospital and get beds."

The current level of infection with the 'flu virus is around 80 cases per 100,000 people a week.

The last severe outbreak in 1995 recorded 230 cases per 100,000 a week and a major epidemic in the 1960s caused 1,180.