Dr Janice Lo, consultant in health protection for London at the UKHSA, said: “At this stage, a common source has not yet been confirmed, but we are undertaking further investigations, with partners, that will provide us with more information.
“Legionella bacteria is widely distributed in the environment and can multiply in large water systems within buildings.
“It is important that water systems are properly maintained with regular testing and treatment to ensure that bacteria do not multiply and cause a risk to health.”
Patients and their families are being interviewed to retrace their steps and this information is being cross-referenced for potential infection sources.
The managers of cooling towers and large water systems within the affected areas have been instructed to urgently treat their system with high doses of chlorine to kill any potential contamination, the UKHSA added.
Signs and symptoms of Legionnaires’ disease include a flu-like illness with muscle aches, tiredness, headaches, dry cough and fever, and people with these symptoms should call NHS 111 or their GP for medical advice.