“Hundreds” of dead fish have been spotted in the River Lea around Clapton over the bank holiday weekend, sparking an investigation by environment chiefs.

The source of the pollution is not yet known, but the Environment Agency (EA) has denied the deaths were caused by water run off from the huge warehouse fire in Tottenham on Thursday night.

Workers have set up equipment at Pymmes Brook, the source of a devastating oil spill last year, to increase oxygen levels in the water.

Gerard Hastings is moored in Clapton and woke up on Saturday to see the dead fish floating down the river by Walthamstow Marshes.

"Something major has obviously happened up river," he said. "I think it's a serious issue."

The EA said yesterday the main pollutants had passed and no further deaths were expected. But they did say carcasses would continue to appear for a few more days, and that it was the responsibility of the Canal and River Trust to remove them from the water.

Last year's oil spill devastated wildlife, stranded boats and turned much of Hackney's canal network black. In March this year the authorities, who had been criticised over their handling of last year's disaster, were praised for their response to another oil slick in the same location.

Lea Bridge local and Hackney Greens activist Heather Hampson said: "It's awful to see hundreds of dead fish in the River Lea.

"We are very concerned about the impact this will have on the wildlife of the area.This latest disaster comes hot on the heels of the oil slick last year, which was largely left to volunteers, including members of Hackney Greens, to clean up.

"If, as suspected, this is another pollution event, we would really like to see the Environment Agency and other official agencies take urgent action to clean up the River Lea and put in place better measures to protect it in the future."

Following last year's criticism, the EA set up a first-of-its-kind dedicated website that allows anyone to sign up for email notifications for significant and major environmental incidents within the Lower River Lea.

An Environment Agency spokesperson said: "We are investigating the source of the pollution. Our officers have placed equipment at Pymmes Brook to increase oxygen levels in the water, and continue to assess what has happened."