But leaked documents suggest the EA failed to investigate this incident thoroughly or challenge the water company’s account of events.

The agency’s attending officer also tested the water on the lake shore, rather than from where the sewage had been discharged, and he reported “no visual impact found”.

The Environment Agency investigation did not work out the total duration or volume of the sewage pumped into Windermere.

It subsequently categorised the pollution as a “minor” incident and the only enforcement action taken was a routine “site warning” issued three weeks later.

The investigation appeared to be closed.

However, after the BBC questioned the Environment Agency about the incident, a spokesperson said it was now investigating.

“We are undertaking a thorough investigation into the incident which involves examing further evidence from United Utilities. If any water company is found to be in breach of an environmental permit the Environment Agency will take the appropriate enforcement action up to and including a criminal prosecution.”