Thames Water was fined more than any other company in England and Wales for creating pollution last year, the Environment Agency (EA) has confirmed.
The water company received penalties totaling £128,000 across the year, the agency revealed in its Spotlight report on business.
Responding, the firm stressed that it was working hard to improve its performance in relation to pollution incidents.
Other major water firms were also named and shamed in the top ten list of polluters, including Southern Water, Severn Trent and United Utilities.
The EA did however reveal that water providers had become better at reporting incidences of pollution they were responsible for.
The agency said that 32 per cent of pollution incidents involving water firms were reported by the companies themselves in 2005, compared to 28 per cent in 2004.
The country's second most prolific polluter, measured according to the level of fines incurred by companies, was the waste firm Sundorne Products, with the Aberystwyth-based company incurring penalties totaling £97,750 in 2005.
Third on the list was the chemical company Robinson Brothers, which was fined £63,000 over the year.
The EA revealed that there had been a 16 per cent increase in the number of companies it prosecuted for causing environmental pollution in 2005.
Overall, 317 businesses were prosecuted through the year, resulting in fines totaling £2,742,250.
The number of industrial incidents that caused major environmental damage also increased by seven per cent, with the waste, water and farming sectors responsible for three quarters of serious pollution incidents.
Even the EA itself was not immune from prosecution, with a £7,500 fine imposed on the regulator this year following a major pollution incident involving one of its subcontractors.
"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," the agency said in its report.