Up to 200,000 people a week are being struck down by a violent stomach bug sweeping Britain, in what the Health Protection Agency has confirmed is the worst outbreak of norovirus in five years.

So far this winter around two million people have fallen ill with the virus, more commonly known as winter sickness bug. This is twice the number of cases seen last year and the highest number since 2002.

But the HPA gave warning that the true number of norovirus sufferers could be much higher. Although doctors across the country have reported that they are being inundated with cases, many people will not have visited their GP.

Professor Steve Field, chair of the Royal College of GPs, said the number of new cases each week could be more than 200,000.

“Surgeries and hospitals have been swamped with people wanting advice,” he said.

The Royal College of GPs warned those infected with norovirus, which causes vomiting and diarrhoea, not to return to work or school as the holiday period comes to an end. They should also stay away from hospitals and surgeries and instead phone in for medical advice to avoid spreading the bug.

Professor Field said: "Wash your hands regularly so you don't infect anyone else and stay at home two days after the symptoms have gone."

The bug is easily transmitted through contact with an infected person, contaminated food or water, and even contaminated objects or surfaces.

Earlier this week, the virus swept through the Queen Victoria cruise ship as it headed to the Canary Islands, infecting around 80 passengers. More than a dozen hospitals have reported cases of norovirus and NHS Direct said that vomiting and diarrhoea were among the top five complaints it has received in the last two months.

A report from the HPA said: "The number of cases reported this year surpasses all other recent years, except 2002, which was the most severe season recorded and was associated with the emergence of a new virus type.

"Outbreaks affect health care facilities worldwide and may cause massive disruption to providing care, substantial economic loss, and, according to some reports, mortality in vulnerable patient populations."

It is estimated that a norovirus outbreak could cost the NHS around £100 million as operations are cancelled and staff call in sick. It could cost the economy a further £12 million a week in lost productivity.

Norovirus is the most common cause of infectious gastroenteritis in England and Wales. It is not normally dangerous and most people can expect to make a full recovery within 48 hours. But in very old and very young patients, it can cause complications from dehydration.

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