Frogs and invertebrates have been found dead after debris from a nearby construction site was discovered in a waterway, campaigners have claimed.

Kim Coleman, who leads a campaign against the Cat Hill housing development, says she found bricks, dust and chemicals in a stream leading into Pymmes Brook at Oak Hill Nature Reserve.

She believes the pollution has come from the development on the former Middlesex University Campus in Cockfosters, where developer L&Q has permission to build 231 homes.

The campaigner told the Times Series how a concerned ranger at the nature reserve asked her to come and look at the damage.

She said: “I took photographs of what are clearly bricks, dust and chemicals coming from the site. We knew there were chemicals because of the odour.

"We found four frogs dead and the ranger informed me that all the invertebrates would have been wiped out as a result of this. Something has to be done now.”

London Assembly Member for Barnet and Camden Andrew Dismore has sent a letter to Environment Agency chief executive Dr Paul Leinster, demanding he uses his powers to halt construction on the site while Pymmes Brook is tested for contaminants.

If tests prove there is contamination, Mr Dismore has asked that enforcement notices are issued to L&Q.

Mr Dismore said: “It’s killed off the frogs and bugs and creepy crawlies that are vital to the brook, and all that pollution will find its way into major water sources.

“It’s about time the enforcement authorities pull their finger out and take action before what’s left is gone completely.

“What is going on at Cat Hill is environmental vandalism. The Environment Agency has the powers of enforcement, it just needs to use them.”

The Times Series is waiting for a comment from the Environment Agency and L&Q.