The company, based in Huntingdon, Cambridgeshire, admitted discharging sewage effluent into a tributary of the River Great Ouse at Clenchwarton.

At King's Lynn Magistrates Court on Tuesday Anglian Water was fined £17,500 and ordered to pay £1,856 costs.

The court heard the Environment Agency, which brought the prosecution, visted King's Lynn Sewage Treatment Works on 27 March 2002.

The visit followed a complaint from a member of the public that a discharge from a pipe on the site was turning a watercourse black.

Officers found a continuous trickle of untreated sewage discharging from the works had polluted the tributary to its outfall into the Great Ouse.

Anglian Water asked that a similar offence of discharging trade effluent, red dye, on 22 April be taken into account by the court.