Almost three million people may have been affected by the norovirus vomiting bug this winter, new figures suggest.

Cases of the virus are sweeping the country and are partly to blame for three hospitals being placed on red alert, an NHS manager said today.

Lincoln County Hospital, Grantham and District Hospital and Boston's Pilgrim Hospital have all been placed on their own highest state of alert after patient numbers rocketed 20%.

The Health Protection Agency (HPA) confirmed that laboratory reports of norovirus in England and Wales in the last three months of last year were more than double the figure for the same period in 2006.

There were 1,922 lab reports of the bug from September to December compared with 935 cases the year before, the HPA said.

In December the HPA said laboratory reports reflected "a tiny fraction" of all infections, estimating that for every case another 1,500 went unreported.

This equates to about 2.9 million people affected so far this winter.

The HPA said the norovirus season had started "uncharacteristically early", with a greater number of cases nationally from the first week of November.

Improved reporting and testing may have contributed to the increase.

Last night Grantham and District Hospital admitted five patients with norovirus while another three contracted it overnight. Five staff at the hospital have already gone down with the virus today.

The NHS trust that runs the three hospitals has been forced to open about 85 beds in day care units, a specialist eye ward and a ward reserved for the elderly.

George Briggs, general manager for emergency care for Grantham, Lincoln and Louth, said: "We have had a 20% increase in the number of people being referred to us by GPs with illnesses. We have had some with cold and flu and we have had some with the norovirus.

"We expect an increase in the winter, we always do, hence we put some beds to one side. We didn't expect a 20% increase, I have to say."

United Lincolnshire Hospitals NHS Trust placed the three hospitals on red alert yesterday after it saw a surge in patient numbers.

The only higher alert is black and that can only be put in place by the strategic health authority that covers Lincolnshire and its neighbouring counties.

Doctors have estimated more than 100,000 people a week are catching the stomach bug, which causes diarrhoea and vomiting.

Norovirus frequently causes outbreaks in places where people congregate, such as schools and cruise ships.

Many cases also occur in hospitals and may cost the NHS more than £100 million a year during epidemics.

Norovirus is the most common cause of infectious gastroenteritis in England and Wales.

The virus is easily transmitted by contact with an infected person, by consuming contaminated food or water or contact with contaminated surfaces or objects.

According to the HPA, symptoms will begin around 12 to 48 hours after infection and will usually last 12 to 60 hours.

Most people make a full recovery within one or two days, however some (usually the very young or elderly) may become very dehydrated and require hospital treatment.