AN urgent investigation has been launched by the Environment Agency after 10,000 fish died in an urban lake.

A hotline number has been opened by the agency which said it was baffled by the deaths but was looking at the possibility of a mystery pollutant getting into the water.

Fisheries officers were first called to Killingworth Lake, on the outskirts of Newcastle, on Monday afternoon when an angler reported dead fish in the lake.

As many as 6,000 roach and up to another 1,500 perch, bream and carp have already been collected from the lake and the agency expects expect another 2,500 fish to be recovered in the next few days.

Water samples from the lake, fast-tracked through the Environment Agency's laboratory in Leeds, have shown that the pH level of the water was average and while the level of oxygen in the water was low, it was not low enough to have caused such a major reaction.

Environment Agency spokesman Graham Siddle said: "We've never seen anything that has had such a devastating but localised effect like this.

''More than six square kilometres of land drain into the surface water system that eventually empties into this lake, so we have a lot of ground to cover.''

Killingworth Lake, a popular urban fishery often used as a venue for tutoring budding anglers, has recently had improvement work done, including a habitat creation project.

Mick Donkin, ecological appraisal officer for the Agency, said: ''It's ironic that the success of this work has probably added to the death toll. We're picking up lots of young fish - between one and two years old.

The Environment Agency's incident hotline number is 0800 807 060.