PASSENGERS due to board the luxury Southampton-based cruise ship Caronia have been warned they could risk catching a nasty bug.

Hundreds of holidaymakers expecting to leave for the Baltic on Wednesday have been sent letters saying they could catch a 24 to 48-hour stomach bug causing gastro-enteritis.

Passengers packing for the two-week cruise which takes in Russia, Germany and Denmark are now facing the prospect that they could become violently ill with vomiting, diarrhoea and stomach cramps.

Elderly would-be passengers with a doctor's note are allowed a refund - but the rest of the ship's 688 holidaymakers are now brushing up on Cunard's tips to avoid catching the illness.

A Cunard spokesman has admitted this is not the first time the ship has been struck down by the digestive viral infection.

In May an outbreak affected 170 passengers and crew on the ship and this is the third cruise where Cunard bosses have been forced to send out warning letters to holidaymakers.

"I wouldn't say this was common but it does happen on ships where you have a lot of people in an enclosed area," said the spokesman. "Person to person diseases such as this are spread quickly."

He said there were currently 20 passengers on the Caronia suffering from the infection, but no new cases had been reported in the past few days.

On the last cruise, from Spain to Southampton, 15 people demanded their money back. But it is not yet known how many plan to drop out of the coming 14-day voyage, which costs up to £12,000 for a luxury penthouse cabin.

The spokesman added: "We are not obliged to tell people about the illness but we feel it's only fair to warn them and give them the option not to come.

"We believe the responsible way for a cruise company to behave is to advise people in advance rather than allowing them to turn up and then find out."

The viral infection can be spread easily, particularly if people do not wash their hands properly.

Cunard's website boasts that Caronia is "a haven of sophistication and relaxation. There's so much space in which to unwind that the stresses of the outside world soon melt away."