Thames Water was fined more than any other firm in England and Wales for pollution last year, the Environment Agency has said.
Serious environmental damage incidents rose in 2005
The company, which received penalties totalling £128,000, was joined by other water giants Southern, Severn Trent and United Utilities in the top 10 list.
Overall, major environmental damage rose 7%, with the waste and farming sectors also among the worst offenders.
Thames said it was working hard to improve performance on pollution.
The Environment Agency said 32% of water firm pollution incidents were reported to it by the companies themselves last year, up from 28% in 2004.
It said it was developing a plan to improve the sector's pollution record further.
Waste firm Sundorne Products (Llanidloes) came in second place in the list detailing fines, with penalties of £97,750, followed by chemical firm Robinson Brothers, which was fined £63,000.
One of the Environment Agency's own subcontractors caused a major pollution incident in 2005 which resulted in a prosecution and fines for both, its report noted.
"This illustrates both the need for all businesses to ensure the good environmental performance of their supply chain, and the damage that environmental harm can do to your reputation," it said.