For the third year in a row Anglian Water has received a 2 out of 4 star rating from the Environment Agency.
The low rating was part of an environmental performance assessment and the regulator has told Anglian Water that it must do better.
The report says the water company was responsible for more than 300 pollution incidents including 11 serious ones last year.
Anglian water was one of only two UK water companies to receive two-star rating.
Earlier this month Anglian Water was fined £50,000 for failing to provide data requested by the Environment Agency as part of an investigation by the regulator into ten water companies and potential violations at more than 2,000 waste-water treatment plants.
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