The Environment Agency said the algae levels in the Flashes are "sufficiently above its warning threshold and should therefore be regarded as posing a potential hazard to human and animal health".

Illnesses — including eye irritation, vomiting, diarrhoea, skin rashes, muscular pains and fever — have affected people who have swallowed or been swimming in infected water. The algae can also harm animals.

The algae rises to the surface of the water as a scum and looks like
blue-green paint.

Notices are in place around the popular beauty spot warning members of the public not to swim in the water.

Martin Kimber, Wigan Council's director of environmental services, said: "The flashes are popular places but contact with the algae should be avoided.

"People should not swim in the water and should keep their pets well away from it."

It is the first sighting of the algae this year as the organisms have thrived in the recent warm weather.

Environmental health chiefs said levels of the algae fluctuate, meaning it can appear one day, disperse in the wind the next, but come back at any time.

This pattern is peculiar to the weather conditions at this time of year, but the algae will disperse naturally when the cooler weather arrives.

Blue-green algae is found naturally in many inland waterways, while in lakes, ponds and pits the warmer summer weather is ideal for its growth.