The Environment Agency has begun a criminal investigation after identifying the source of the pollutant which resulted in thousands of fish dying in Doe Park reservoir in Denholme last month.

It comes as the agency warns residents and businesses to dispose of chemicals carefully, and not to pour them down drains.

They were called to the reservoir five weeks ago after reports of dead and distressed fish at the site, and found 2,500 dead roach, bream, perch and pike.

Following an initial examination, fisheries officers said they believed the cause of the pollution was organic matter that had been washed into the water through recent flooding.

Ten barrels of hydrogen peroxide were pumped into the reservoir on Monday to help break down organic matter which can cause poor water quality.

At the time oxygen levels in the water were found to be good.

Shortly afterwards the Environment Agency said its investigations, along with support from the local community, had revealed the potential source of the contaminant.

Now the Telegraph & Argus has learned that a criminal investigation has been started.

Louis Harvey, environment management team leader for the Environment Agency, said: “We have identified the source of the pollutant which caused this serious incident, and have now started a criminal investigation.

“Levels of pollutant in the reservoir have decreased greatly, and we have been keeping Bradford Council’s Environmental Health department informed.

“Our staff continue to check the reservoir regularly.

“It’s important that chemicals are disposed of carefully, and not poured down drains.

“Residents and businesses wanting to dispose of chemicals should contact their local authority, who may be able to help or advise.”

Bradford Council, which operates a water activities centre at the reservoir, was forced to postpone an open day which had been planned as the centre remained closed for three weeks. It has since reopened for activities and the rearranged open day was held at the weekend. The reservoir is a compensation reservoir and is used to top up river levels.