A 15-MILE stretch of the River Irwell has been poisoned after a suspected pollution incident which is thought to have originated in Rossendale.

Conservation experts say invertebrates along the waterway have been 'wiped out' beyond the water treatment works at Ewood Bridge, possibly by the dumping of a harmful pesticide.

An investigation has been launched by the Environment Agency after the incident was reported to the watchdog by the Mersey Basin Rivers Trust, which monitors the Irwell.

Mike Cuddy, the trust’s chief executive, said: “There has been a 15-mile total wipe out of river invertebrates, from a point just south of Rawtenstall to the confluence of the River Roch in Radcliffe.

"There has been a 90 per cent reduction in numbers from Radcliffe down to the city centre of Manchester, where the Irwell joins the Manchester Ship Canal.”

It is feared the pollution could stretch beyond that radius, once checks are made of the ship canal and River Mersey later.

Sampling teams from the trust, taking part in the national Riverfly Partnership monitoring initiative, first noticed the watercourse had been affected in the Irwell Vale area.

Trust bosses believe the pollutant, which resembles a potent agricultural pesticide called Chlorpyrifos which devastated the River Kennet in Wiltshire and Berkshire four years ago, has been illegally dumped in land drains before reaching the Irwell.

Mr Cuddy said: “We do not think the water company is at fault for this catastrophic incident, rather that an irresponsible individual or business in the sewer network area around the treatment works has illegally disposed of toxic chemicals down the drains.

“This toxin has made its way through the sewer network, through the sewage treatment beds and then into the river via the water treatment works outflow.”

Similar sampling locations, before Ewood Bridge, have not been affected, the trust said.