The agency is investigating how an estimated 5,000 gallons of slurry came to enter Lockholme Beck - a tributary of Scandal Beck, which eventually runs into the River Eden at Kirkby Stephen - killing all signs of water life in the worst- affected stretch.

The Environment Agency discovered 49 dead trout and up to 400 dead bullheads and stone loach over a 3km stretch of the beck after being alerted to the incident on Saturday.

Numerous dead eels were also found, although no figure has been given on the number killed. No reports have been made of affected fish in other nearby water-courses.

The Environment Agency was alerted to the incident by a member of the public who spotted the potential pollution on Saturday and called the free 24-hour pollution hotline.

The slurry, which the Environment Agency says is believed to be from a store at a nearby farm, contains toxins which strip oxygen from the water, therefore suffocating fish.

Claims have been made that the slurry was released in a deliberate act by trespassers.

A spokesman for the Environment Agency said that investigating officers were looking into all possibilities of how the incident might have occurred, but would not speculate until the investigation was complete.

Investigating officer lan Creighton said the incident had a severe impact on the watercourse.

He said: "Slurry has a devastating effect. This is a very serious pollution that has killed hundreds of fish. In the worst-affected part of the beck, we believe every fish has been wiped out."

The Environment Agency operates a 24-hour emergency hotline so that members of the public can report suspected pollution incidents. The number is 0800 807060.