Thames Water has apologised after being fined £12,323 for polluting a tributary of the River Thames twice in two years.
The Reading-based company pleaded guilty to two offences under the Water Resources Act 1991 on Wednesday.
The court heard that in January 2008, the firm's pump at Buckingham Avenue in Slough, Berkshire, failed to divert a diesel spill.
Thames Water was also found guilty of leaking raw sewage into Haymill Stream and Chalvey Ditch in December 2009.
Thames Water Utilities Ltd was fined £4,000 for each offence and ordered to pay £4,323.08 costs to the Environment Agency at Bracknell Magistrates Court.
The Environment Agency said in the 2008 incident, 4,500 litres of diesel was lost from a split pipe within the pump room of a data-processing centre based in Slough Trading Estate.
The oil flowed out of the unit's drains into the surface water road drains of the trading estate, discharging into the Buckingham Avenue interceptor which belonged to Thames Water.
'Very much regret'
It added that the pump within that interceptor did not work because the sensors that activated it were faulty.
As a result of the spillage, about 50 fish in the Chalvey Ditch were killed.
In the 2009 incident, a member of the public reported chemical smells around the Chalvey Ditch.
Samples of the water taken revealed the stream had been polluted by organic matter, the agency said.
On that occasion, Environment Agency officers traced the pollution to the Buckingham Avenue interceptor, where the drains that should have taken the effluent to the foul sewer pumps had become blocked with silt and debris.
Neil Martin, the agency's investigating officer, said: "Both these incidents could have been avoided with a comprehensive maintenance plan and better understanding of the Buckingham Avenue site on the part of Thames Water."
In a statement on Thursday, the firm said: "We very much regret the incidents in January 2008 and December 2009.
"While the pollutant in each incident came from companies on the Slough Trading Estate, we accept our part of the responsibility."