Two elderly British holidaymakers have tested positive for the Legionella bacterium after being evacuated from their luxury Baltic cruise to a Swedish hospital.

The two were among five female and two male passengers, all in their 70s and 80s, transferred to hospital in Stockholm last Friday from the Fred Olsen Cruise Lines ship Black Watch after falling ill with “pneumonia-like symptoms”.

The ship returned to Britain early after the evacuations, docking in Dover last night, two days ahead of schedule. Tests on the Black Watch have so far proved negative for the bug, and passengers disembarking from the liner pointed the finger at Russia as the source of the infection.

The cruise included stops at ports in Russia, Estonia, Finland and Sweden, and had been due to continue to Lapland until its itinerary was cut short.

Most of the 756 remaining passengers were smiling when they left the ship at around 9pm last night, despite losing two days of their holiday. Some of them had paid £6,000 for the 17-day trip, but the cruise company has offered them a 25 per cent refund.

Most Fred Olsen customers are aged over 60, according to spokeswoman Wendy Hooper-Greenhill.

The seven passengers who fell ill were seen by the ship’s doctor and taken to hospital in Stockholm, with two testing positive for Legionnaires' disease.

Six of the passengers have now flown brought back to England by their health insurance providers, but one woman remains in Sweden. She is not suffering from Legionnaires’ and has an unrelated complaint, Ms Hooper-Greenhill said.

Two of the six passengers were taken to the William Harvey Hospital in Ashford, Kent. Dr Mathi Chandrakumar, director of the Kent Health Protection Authority, said it was unclear what was wrong with them. It is understood that the other four are also at hospitals in the UK, but this morning it was not clear where.

Ms Hooper-Greenhill 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.

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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