Thames Water have been fined more than £3m after admitting polluting rivers.
The company, which supplies one in four people in Britain with water, pleaded guilty to four charges.
It was fined £3.3m at Lewes Crown Court on Tuesday.
The court heard that an estimated “millions of litres” of undiluted sewage was pumped into the Gatwick Stream and River Mole between Crawley, in West Sussex, and Horley, in Surrey, on 11 October 2017.
The hearing was told that the spill turned the water “black” and killed more than 1,000 fish.
During the first day of the hearing on 3 July, the court heard how a storm pump at Crawley Sewage Treatment Works site was unexpectedly diverting sewage to its storm tank for 21 hours and went “unnoticed”.
Prosecutor Sailesh Mehta estimated that untreated sewage was spilling into the river for six and a half hours after no alarm was raised.
Eyewitness accounts read in court said how they saw the river turn “black” and “grey”, with “huge numbers of dead fish” visible in the water.