Norovirus causes sudden vomiting and diarrhoea.

People struck down by a stomach bug sweeping the UK have been urged not to go back to work.

Doctors estimate more than 100,000 people a week are catching norovirus, which causes diarrhoea and vomiting.

Workers need to remain at home for 48 hours after the symptoms have gone and stay away from surgeries and hospitals, the Royal College of GPs warned.

The winter bug has forced the closure of at least 56 hospital wards across England and Wales, the BBC has learned.

The Worcestershire Acute Hospitals Trust says it is cancelling all non-urgent operations until 9 January because of what it calls the "unrelenting pressure" caused by the virus.

The Royal Oldham Hospital has 11 wards closed which is more than a quarter of its total and Birmingham Heartlands Hospital says six areas of the hospital are affected.

The Health Protection Agency confirmed the number of cases this year is the highest since 2002.

Norovirus - also known as winter vomiting disease - is the most common cause of infectious gastroenteritis in the UK.

Illness can occur at any age because immunity to it is not long-lasting.

It is not normally dangerous but the very young and very old are most at risk of complications from dehydration.

The bug can be spread by contact with an infected person, through contaminated food or water or by contact with contaminated surfaces or objects.

Outbreaks are common in hospitals, nursing homes, schools and cruise ships.

'Stay home'

Professor Steve Field, chair of the Royal College of GPs said its surveillance unit in Birmingham had confirmed the number of cases was the highest in five years.

He said the number of new cases each week may even top 200,000 across the UK.

"Surgeries and hospitals have been swamped with people wanting advice."

He advised those affected to stay at home, drink lots of fluids and take paracetamol.

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

Dr Darren Simpson, a GP in Bradford said they had seen a large number of people with norovirus in recent weeks.

"Very few people follow the advice of staying away from work however due to unsympathetic and often nagging bosses.

"It's the worst time of year to catch it as a lot of places are understaffed due to leave and bank holidays so there is increased pressure to attend work if you can."

Early outbreak

In December, the Health Protection Agency warned the norovirus season had started particularly early.

They estimate that in epidemic seasons, noroviruses may cost the NHS in excess of £100m per year because of ward closures.

In 2007, the number of cases reported to the HPA in England and Wales from early September to early December were double those seen in 2006 - 1,325 compared with 685.

The actual number of cases is much higher as most are not reported - perhaps as much as 1,500-fold.

NHS Direct said calls about vomiting and abdominal pain had been the number two reason for people calling over the Christmas period - in keeping with reports of increased rates of norovirus.

A total of 1,122,874 people contacted the NHS helpline over the 11-day Christmas and New Year period - a 61% increase on last year.

Dr Mike Sadler, NHS Direct chief operating officer, said: "Call patterns have been markedly different from our previous experience."