Water chiefs have been criticised after aqueduct levels plummeted at the weekend, leaving wildlife to perish – “cooked slowly by the sun’s heat”.

It is estimated 20,000 fish may have died along a one-mile stretch of the New River in Stoke Newington.

Wildlife volunteer Matthew Sherwood realised the fish were in danger last Wednesday – and reported the problem to the GLL Better team that runs water activities at the West Reservoir. But he claims nothing was done to remedy the problem until Monday – by which time it was too late.

“Most of the fish were belly-up by Sunday and the river flow had reversed,” he said, “meaning only three-quarters of an inch of stagnant oxygen-depleted water remained.

“No action was taken to fix or restart the pump until Monday morning, so in the meantime all the water drained from the river system leaving all of the wildlife to die. On Monday after the pump had been working for an hour or so, the top section had six or seven inches of fresh water but the fly-infested carcasses of the fish were floating down to the dam wall.

“The restock of the fish will cost thousands – a lot more than the cost of getting an engineer out to fix the pump or put a stand-by pump in place. It’s an absolute disgrace.”

A GLL spokesman said: “We have been in communication with Thames Water as recently as last week over the issue of falling water levels during the dry spell. Unfortunately last week water levels reached a low level that may have impacted on wildlife. We are urgently seeking assurances from Thames Water they will rectify the problem as soon as possible.” But Thames Water pointed the finger back at GLL, saying the leisure firm was responsible for maintaining the pipes and pumps whose failure had sparked the crisis.

A spokeswoman said: “When we were contacted last week to increase the flow of water to the reservoir and adjoining section of the New River we responded straight away.

“However, we were advised that the pipes and pumps belonging to GLL were not working properly meaning our actions were not effective in increasing the water levels.

“We understand the issues with those pipes and pumps have since been resolved and we’ve offered our support to reduce the risk of future problems.”

But she also said Hackney Council, not Thames Water, was responsible for the West Reservoir and the New River where it passes through the borough.