More than 750,000 people could be affected by an outbreak of norovirus in the UK, the Health Protection Agency (HPA) has claimed.
Among those affected are passengers on board a cruise liner in the Baltic, who have been confined to their rooms and warned not to leave the ship.
The Daily Telegraph has reported that 300 of the 1,843 passengers on the Oriana were affected by the highly contagious bug, which causes vomiting and diarrhoea.
A spokeswoman for P&O Cruises said that six people on board have active symptoms today, but she was not able to confirm how many people have been affected in total.
There have been 2,630 confirmed reports of norovirus so far this season, but for every reported case there are likely to be a further 288 unreported sufferers, the HPA added.
It means 757,440 people could be affected by the stomach bug - a 72% increase so far this season.
A spokesperson for the HPA said: "Laboratory confirmed reports represent only a small proportion of the actual amount of norovirus activity in the community, because the vast majority of affected people do not access health care services as a result of their illness."
The spokeswoman for P&O cruses
said: "The safety and comfort of passengers and crew is always our number one priority.
"As is currently standard procedure across our fleet, all the ship's passengers were provided with a precautionary health notice advising of widespread norovirus activity and the health measures to avoid contraction and spread, both on board and whilst ashore."
She added that enhanced sanitation protocols had been implemented to minimise the spread to other passengers.
Suffering passengers have been asked to isolate themselves in their rooms and not leave the boat for day trips.
Any medical charges incurred as a direct result of the outbreak will be covered by the company, the spokeswoman confirmed.
A spokesman for the HPA said there was no indication as to why there are more cases of norovirus this year than last.
However, the HPA did report a drop in cases over the last few weeks, with 236 cases reported during the week ending December 2, compared with 327 in the week ending November 25.
John Harris, an expert in norovirus at the HPA, said: "Our figures show a small drop in the number of confirmed cases over the last couple of weeks.
"We cannot read too much into this at present as this is typical of the norovirus season, where we see a series of sharp rises and falls in activity between October and April with the bulk of cases usually occurring between January and March.
"People should be vigilant in their hygiene and we would like to remind anyone who has typical symptoms suggestive of norovirus infection to avoid visiting friends or relatives in hospital or care homes.
"Norovirus infection in hospitals is very disruptive as it can lead to ward closures.
"Having a norovirus infection is very unpleasant but it is short-lived and most people will fully recover in a couple of days.
"Make sure that you or anyone you are caring for takes plenty of fluids to avoid dehydration.
"Over-the-counter medicines can also be useful in reducing headaches and other aches and pains".