Elderly holidaymakers were being treated in a Swedish hospital last night after it was feared that they had caught legionnaires’ disease on their luxury cruise.

The seven passengers, all in their 70s and 80s, were admitted to hospital on Friday after falling ill with “pneumonia-like symptoms” on a Fred Olsen cruise ship. The condition of all of them is said to be stable.

The ship, the Black Watch, is now carrying her 750 passengers back to Britain instead of continuing her voyage to Lapland. She is expected to arrive in Dover this afternoon.

The five women and two men patients are expected to spend the next few days in hospital.

Holidaymakers paid up to £8,000 for the cruise on board the Black Watch, which Fred Olsen describes as “resplendent”.

Wendy Hooper-Greenhill, a spokeswoman for Fred Olsen, said that the vessel would undergo a “deep-cleansing regime” to ensure that any infection aboard was killed. “If there is something lurking there, that will certainly knock it on the head,” she said. The ship’s pools and whirlpool spas have been closed as a precaution, she said, on the advice of Swedish health authorities.

The Black Watch was due to return to Dover on Wednesday, when she was scheduled to embark on another cruise. She had visited several ports in northern Europe, including St Petersburg, Estonia and Finland, before arriving in Stockholm.

Last year a ship operated by the same company was struck twice by contagious viruses. In June 2006, 116 passengers were confined to their cabins on board the Black Prince. Soon afterwards 100 passengers suffered from a vomiting illness during a trip to Greenland and Iceland.

Legionnaires’ disease is a potentially fatal form of pneumonia that usually affects middle-aged or elderly people. The symptoms are similar to those of flu, and include feeling feverish, muscle pain, headaches and a cough. It can be treated successfully with antibiotics.

Critics have said that Fred Olsen’s brisk 12-hour turnaround puts passengers at risk by not giving enough time to disinfect the ship, but the company insists that it leaves sufficient time for a thorough cleaning.

This year more than 300 passengers and crew on the Queen Elizabeth 2 caught the same virus that blighted the Black Prince.

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