South West Water (SWW) has achieved just two stars out of a possible four for an "unacceptable level of performance", the Environment Agency has said.
The agency's annual report said only one of the nine major water companies in England was performing at the expected level, with most likely to miss 2020 targets.
SWW was also given a red rating for pollution incidents for "consistently demonstrating unacceptable performance".
Trade body Water UK said the report was "disappointing" and the situation was "never black and white".
SWW said it recognised there was "still more to do and we have an action plan in place to continue to drive improvement".
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