Thames Water have headed the Environment Agency's 1999 table for worst environmental offenders, with eight prosecutions leading to 79,000 in fines.
Environment minister Michael Meacher says courts are being too soft on polluters and has asked the Home Office to probe.
Fines have been derisory in many cases penalties of up to 10,000 are nothing to a major company. "You have to have a six-figure fine if it is going to have a detrimental effect", Meacher said.
A Thames Water spokeswoman said: "We regret our position in the league table. But it takes no account of the fact we are the largest water and sewage company."
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