More than 750,000 people could be affected by an outbreak of norovirus in the UK, the Health Protection Agency (HPA) has said.

There have been 2,630 confirmed reports of norovirus so far this season, but for every reported case there were likely to be a further 288 unreported sufferers, the HPA said.

It means 757,440 people could be affected by the stomach bug – a 72% increase so far this season.

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 healthcare services as a result of their illness."

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.

It has been reported that up to 400 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 six people on board had active symptoms on Thursday, but she was unable to confirm how many people had been affected in total.

The bug struck passengers after the vessel departed from Southampton last Tuesday for the 10-day journey.

A spokeswoman said: "The safety and comfort of passengers and crew is always our No 1 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 said enhanced sanitation protocols had been implemented to minimise the spread to other passengers.

Suffering passengers had 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 would be covered by the company, the spokeswoman confirmed.

A spokesman for the HPA said there was no indication as to why there were more cases of norovirus this year than last.

However, the HPA did report a drop in cases over the past few weeks, with 236 cases reported during the week ending 2 December, compared with 327 in the week ending 25 November.

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."