TOXIC blue-green algae has formed over the surface of Pennington Flash — just days before it hosts the swimming section of Bolton’s huge Ironman Triathlon.

Large sections of the flash are covered in the poisonous and unsightly scum, and yellow hazard signs have been placed around the Leigh reservoir.

The danger signs read: “Warning! Blue-green algae are present in this water. They may produce a toxic scum. If you see a scum you should avoid contact with it and the water close to it.

“Pet owners should ensure that their animals DO NOT have access to affected water.”

More than 1,400 athletes are due to jump into the reservoir on Sunday, but race organisers maintain that the water is safe.

Race manager Richard Wallace moved quickly to reassure athletes yesterday.

He said: “We’ve had advice from experts, and they say it’s okay. We can almost guarantee that the water is clean and safe to bathe in.

“The council put out blue-green algae warnings when they did a sample, but they’re in every single stretch of water in the country. It’s not something we’re too worried about.”

Blue-green algae, also known as cyanobacteria, are a group of photosynthetic bacteria, commonly referred to as pond scum.

They grow in lakes, ponds, and slow-moving streams when the water is warm and enriched with nutrients like phosphorus or nitrogen.

Algae poisoning can cause stomach cramps, diarrhoea, vomiting, headache, fever, muscle weakness and difficulty breathing.

Experts in the USA, where blue-green algae is also common, say triathletes should try to minimise their ingestion of water and should seek medical attention if they show any signs of illness during or after the event.

Athlete Christopher Murray, aged 39, from Lostock, said: “I know it’s something that’s poisonous, but I just take the view that the organisers have got to worry about it, and I trust they’ve looked into it. I’ll keep my mouth closed. It’s the least of my worries.”

Another athlete, Philip Taberner, from Bolton, said: “I’m aware of it but it’s not been mentioned by the race organisers, so I assume it’s not an issue.”

In August 2008, Clarence Lido, off Walmersley Road, Bury, was drained because council bosses said the blue-green algae was a health and safety hazard.

The Ironman UK Triathlon, which involves a 2.4-mile swim, 112-mile bike ride and a marathon, is on Sunday. It is the second year that Bolton has hosted the event. The swimming section was last year held in Rivington reservoir, but was moved to Pennington Flash this year after United Utilities refused permission to use it again.