The number of sites, all in Edinburgh, rose by two yesterday as health officials worked to establish the cause. The Scottish Government would not release their names , but it is understood they are further from the original south-western zone of the city.

Burton's Biscuits factory in Sighthill, North British Distillery and Macfarlan Smith pharmaceuticals in Gorgie, and Aegon insurance in South Gyle have already been tested.

There are now 23 cooling towers in total being inspected, with results due by the weekend.

A Burton's spokesman said its "water systems are maintained to the highest standard".

Aegeon said it was confident its towers were not responsible while a spokesman for Macfarlan Smith said: "We have a weekly test on site and the cooling towers are monitored by an outdoor company."

North British Distillery did not comment.