The winter vomiting bug norovirus has struck 2.8million people, with health professionals braced for another rise as people return to schools and offices.

The virus - which causes projectile vomiting, diarrhoea, mild fevers and headaches - is striking down more than 200,000 a week, according to official estimates.

Three hospitals have been placed on red alert, while hundreds of wards up and down the country have been closed to new patients as the number of beds being taken up by bug victims reaches critical levels.

Schools have even begun sending warning letters to parents explaining the symptoms while employers are calling on staff to stay away from work 48 hours after they have recovered to stem the spread of the virus.

The rate of new cases being confirmed has reached the levels of reports during the massive outbreak five years ago, when officials announced an epidemic.

Norovirus can prove deadly for vulnerable people, such as children and the elderly. The impact of the bug has been exacerbated by a new outbreak of flu with those most at risk now being given antiviral drugs by their doctors.

NHS Direct, which patients can telephone for health advice, has been inundated with people calling with symptoms of the norovirus.

Helen Young, the clinical director, said: “We are seeing an increasing number of calls about diarrhoea and vomiting. Norovirus is a major issue for the whole NHS right now and we urge anyone who has symptoms to engage in good hygiene to prevent it spreading further and to drink plenty of fluids.”

The number of reports of norovirus is expected to rise over the next six weeks, as children return to school and employees head back to work after the Christmas break.

A spokesman for the Health Protection Agency said it was too early to say if the disease had reached its peak. A statement said: “This season we have seen an increase in reports of norovirus cases, almost double the number reported for the same period last year.

“The self-limiting infection usually only lasts a few days, hence the majority of cases are not reported to the clinician.”

The Health Protection Agency has confirmed that 1,922 laboratory samples tested positive for norovirus.

The agency expects 1,500 cases in the community for every one found in its labs, bringing the total number to 2.8million infected people – or a million new cases each month.

Lincolnshire health officials have placed three hospitals on red alert following a 20 per cent rise in infections over expected levels.

George Briggs, the general manager for emergency care at United Lincolnshire Hospitals NHS Trust, said: “We have had a 20 per cent 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 per cent increase.”

Boots, the High Street chemist, has also reported a 50 per cent rise in the sale of anti-diarrhoea treatments compared to last year.

The Norovirus started to appear a month earlier than expected this year and has already exceeded the levels of the record 2002 to 2003 outbreak, when 935 cases were confirmed by tests.

Over the last four years, Britain has seen the bug arrive and spread quickly in late November to December and then peak in January before falling to insignificant levels by the spring.

However, the number of reports would have to increase dramatically or continue into the summer for an epidemic to be declared.

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