Cup parade fans struck down by Legionnaires'

She said the outbreak, which has claimed the life of a man in his fifties, is possibly the worst north of the Border since the early 1980s.

It is not clear how many of those infected saw Hearts beat Hibs 5-1 in the match, or took part in celebrations that lasted for several days.

Ms Sturgeon said: "Some of the patients were at Hampden. But it's by no means the case that all were involved in the celebrations."

The airborne infection is believed to have been released into the atmosphere by a faulty cooling tower in a plume of vapour. It may have travelled over the south-west of the city for days.

The length of time it takes to test for Legionnaires' means the extent of the outbreak may not be gauged until this weekend when results are ready.

The disease has now hit a suspected 40 people, a rise of eight from earlier in the week, with 21 confirmed cases and 19 suspected cases. There are normally 35 to 40 cases seen annually across Scotland.

Ms Sturgeon, Scotland's Chief Medical Officer Sir Harry Burns and NHS Lothian incident team manager told a press conference the risk to the public from the infection – which can take up to two weeks to incubate – was very low.

The process of gathering information from the confirmed and suspected cases is continuing.

Ms Sturgeon added: "As you would appreciate, that's a complex process because it involves, in many cases, dealing with critically ill patients. NHS Lothian have brought additional public health staff to bear."

Two patients have been discharged from hospital and 12 are being treated in intensive care. They reportedly include Rick Gibb, a father-of-three, who is said to be fighting for his life.

Four sites with a total of 16 cooling towers – reported to be North British Distillery, pharmaceutical firm McFarlan Smith, insurance firm Aegon, and Burton's Foods – and two further unidentified sites, have been chemically treated. These sites are also in south-west Edinburgh, with seven towers that have been treated.

Ms Sturgeon said the evidence so far "underlines the view that the source of this infection is an outdoor community source and not an indoor-specific source, such as would be the case if it was a spa in a hotel". She added: "This is the most significant Legionnaires' outbreak we have had in Scotland for a long, long time, perhaps since the early 1980s.

"It's the cause of concern but it's also resulting in all of the relevant agencies working together very closely, both to manage the outbreak and ensure patients have the right treatment, and to identify the source as quickly as possible."

The towers are the "probable

sources of infection" but others are not ruled out.

Dr Duncan McCormick Consultant in Public Health Medicine at NHS Lothian advised those concerned about possible symptoms to contact a GP or a dedicated NHS 24 helpline.

He said: "There is a very low risk to the public and people should go about their daily business as usual. We remain confident, based on the available evidence, that the general source of the infection has been identified and that the treatment to the cooling towers will prove successful.

"We are expecting cases to continue to emerge over the next few days."

About 40 calls relating to the outbreak have been received by NHS 24 since the weekend.

Legionnaires' disease is contracted by breathing in small droplets of contaminated water. Symptoms are mild headaches, muscle pain, fever, persistent cough and sometimes vomiting and diarrhoea.

Tom Bell, of the Royal Environmental Health Institute of Scotland, said identifying the last time the towers thought responsible were properly inspected would be a key factor of the investigation.

l Ms Sturgeon has withdrawn from the panel on tonight's BBC Question Time to continue dealing with the outbreak. She is understood not to want to be accused of diverting her attention to the issue for a programme in which the referendum is expected to be a main issue.