The entire fish population along a 3km stretch of water that feeds into the River Stour has died because of pollution.

The Environment Agency confirmed that no fish had survived the pollution incident on the Stour Brook in Haverhill, which sent the brook’s oxygen levels plummeting.

An agency spokeswoman said around 2,000 larger fish and thousands more smaller fish had died, while local vets have had dogs admitted after they came into contact with the affected area which runs through East Town Park.

The Environment Agency aerated the stretch of water late on Tuesday and into Wednesday, and oxygen levels have now returned to normal.

Haverhill borough and county councillor Tony Brown grew-up playing in the old East Town Park and said he was “mortified” at what had happened.

He added: “I’ve watched it go from a little stream, year-on-year it’s improved, once it became a park there were a few fish and over the past few years it’s absolutely thrived.

“When I heard what had happened I was absolutely angry and shocked, because I know the work the Environment Agency and the borough have done to make it a great resource, and the stream is fundamentally the main attraction. They’ve ruined a jewel in the crown of Haverhill.”

Mr Brown said that he had also seen kingfishers and an otter along the stream in recent times, describing it as “a haven of wildlife”.

The pollution was so severe it caused the oxygen levels to dip in the River Stour itself, although no wildlife is believed to have been affected.

An investigation has been launched by the Environment Agency into what caused the incident.

Penny Hemphill, water for wildlife adviser at the Suffolk Wildlife Trust, said: “We have a number of species along our rivers now and any incident such as this will have an impact.

“If it’s affected fish for that length, it’s obviously a major pollution incident and that is going to have a knock-on effect. Let’s hope they get it cleared-up as quickly as possible

“Any pollution incident is going to have an affect on the riverside habitat and wildlife, and I’m sure the Environment Agency is doing all it can to clear it up so it has the least impact.

“We’d support them and work with them if they needed us to.”