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Hospitals around the UK have been closing wards to visitors in the hope of preventing the spread of the winter vomiting bug, norovirus.

Birmingham's City Hospital, Maidstone Hospital in Kent, and George Eliot Hospital in Nuneaton, Warwickshire, all have patients with the infection.

At Southampton more than 400 virus-infected passengers disembarked from a cruise ship.

The Health Protection Agency has said a 72% rise in cases was unexplained.

In its latest weekly report, it said there had been 2,630 confirmed laboratory reports of norovirus - up from 1,533 cases reported in the same period last season.

"There are no indications as to why activity is higher this year," it said.

However, it added that there had been a 28 per cent reduction in the number of confirmed laboratory reports in the two weeks ending 2 December.

Experts at the HPA stress that it is normal to see a series of sharp rises and falls in norovirus activity between October and April, with the bulk of cases happening between January and March.

The infection is highly contagious, and involves a sudden onset of vomiting and diarrhoea, with possible temperature, headache and stomach cramps. The illness usually lasts one or two days and there are no long-term effects.

The HPA said that lab reports represent only a small proportion of the actual amount of norovirus activity, as most infected people do not go to a doctor for treatment.

"It has been estimated that for every confirmed case there are around 288 unreported cases," the HPA said.

The HPA advices infected people not to visit friends or relatives in hospitals or residential care homes while ill, to avoid spreading the virus.

Southampton's port health authority confirmed that 417 infected passengers disembarked on Friday after a 10-day Baltic cruise with P&O Cruises.

The cruise company apologized to passengers on Oriana who suffered diarrhoea and vomiting.

And on Saturday, a second P&O cruise ship, Azura - on which there were 10 confirmed cases - arrived back in Southampton following an 11-night tour of Iberia.

The company said it would be "deep cleaned" ahead of its departure later on a 12-night cruise of the Atlantic Islands, including Madeira and Tenerife.

The company again apologized saying it did not "want anyone to be ill, especially not when they are on holiday on one of our cruise ships".

At City Hospital in Birmingham four wards were closed to visitors as a precautionary measure to stop the spread ahead of Christmas. Officials hoped to re-open within a week.

In Kent, three wards at Maidstone Hospital were closed to new admissions and family and friends of patients have been told to stay away.

Maidstone and Tunbridge Wells NHS Trust said 27 patients had norovirus.

Four wards have also been closed at The Queen Elizabeth the Queen Mother Hospital (QEQM) in Margate because of the bug.

NHS Tayside reopened two wards on Friday that were closed to new admissions following an outbreak of norovirus.

The wards were at Crieff Community Hospital and Dundee's Royal Victoria Hospital.

In Wales, visitors have been urged to stay away from five hospitals - Ysbyty Gwynedd, Dolgellau, Colwyn Bay, Glan Clwyd and Wrexham Maelor - to prevent the spread of the infection.