The government agency received a report of oil entering the St Leonards reservoir, popular with anglers, and attended last Friday the site to investigate.

Environment officers found the stream that flows into the reservoir was covered in thick black engine oil.

With help from Southern Water, investigations took place including visits to a number of industrial units in the area, but the source of oil could not be traced.

Operations Delivery staff from the Environment Agency have deployed a number of methods to prevent the oil from entering the reservoir.

They installed two types of boom, one which acts as a barrier and one to absorb the substance. They also installed a fibrous material to float on the surface of the water and absorb the oil.

Environment Agency officer Rebecca Long said: "This could have been a serious situation if we had not acted quickly to prevent further spreading of the oil. The Wishing Tree Reservoir is an important site for wildlife as well as a being a popular place for anglers and for other recreational activities.

"We have been unable to determine the cause of this particular incident, but we will be monitoring the site regularly. Even the smallest amount of oil put down a drain can have a devastating effect on the environment. All waste oil should be disposed of correctly and taken to the local civic amenity site."