The cruise has been cut short and the ship is heading for Dover

A spokeswoman for Fred Olsen Cruise Lines said they showed "pneumonia-like symptoms". They were expected to be in a Stockholm hospital for a few days.

The hospital is awaiting test results and it would be wrong to give the illness a name at this stage, she said.

The Black Watch ship was returning to the UK early as a precaution, she said.

The ill passengers, believed to be five women and one man, are all aged in their 70s and 80s. They are said to be in a stable condition and are expected to remain in hospital for several days.

Company spokeswoman Wendy Hooper-Greenhill added: "It is important to remember that we do not know what the infection is, nor do we know how it was contracted."

She said one of the group had an address in Portugal, but could still be from the UK originally.

The Black Watch is due to arrive back in Britain on Monday.

There are more than 750 passengers on board, almost all of whom - bar a small group of Irish holidaymakers - are British. There are also 329 crew members.

The liner set sail from Dover for a 17-day trip to the Baltic almost two weeks ago. It had been due to return on Wednesday when it is due to leave on another trip.

'Thorough disinfection'

Ms Hooper Greenhill said it was returning early as a "precautionary measure".

Swedish officials have taken samples of water from on board the ship to determine if the holidaymakers had caught the illness on board.

It was not clear whether the disease was Legionnaires', but the ship was to be deep-cleaned after all passengers had disembarked.

"If we get the ship back, we can get all the passengers and people off which will allow us to do a very thorough disinfection and cleansing," she said.

"We do this in any event but it means we can have her in port for 24-48 hours and do a really good disinfection and cleansing regime and if there is anything lurking, it will get whacked."

The company added in a statement: "Fred Olsen regards the health and safety of its passengers and crew as the highest priority and will take all steps necessary to eradicate all trace of the bacteria should it be found anywhere on the ship."

Jerker Jonsson, a Stockholm region infectious disease specialist, told AFP that the group of passengers was in a "stable condition".

But he added: "This is probably a case of Legionnaires' disease."

Before arriving in Sweden, the ship had travelled to Estonia, Finland and Russia.

Last year, another Fred Olsen Cruise Lines ship was hit twice within weeks by a winter virus.

A quarter of the 400 passengers on board the Black Prince in July suffered from a vomiting bug similar to the Norovirus which hit 136 passengers on the vessel in June.

Symptoms of Legionnaires' disease are similar to a flu-like illness with muscle aches, tiredness, headaches, dry cough and fever, leading on to pneumonia.