The documents from United Utilities, the group that manages the waste water network in northwest England, revealed that a telecoms fault on February 28 caused the main pumps to stop.
Emergency back-ups then took over and pumped raw sewage intermittently over 10 hours into the lake, which is located in the Lake District national park, a UNESCO World Heritage site.
As well as being a natural beauty spot that attracts millions of visitors each year and a popular swimming spot, it has long been a muse for artists.
It inspired the poet William Wordsworth, forms part of the title of Oscar Wilde's first successful play and is even mentioned by US pop star Taylor Swift in her song "The Lakes".
It was also the scene of several world water speed records in the 20th century.
United Utilities insists it took urgent action to resolve the incident, which comes with wider concern across the UK about the release of untreated sewage into rivers and the sea.
The company told the BBC that the issue "was caused by an unexpected fault in the telecommunications network in the area, which United Utilities was not notified about".
Pumps usually send sewage to Windermere Wastewater Treatment Works, and United Utilities is only legally allowed to discharge untreated sewage into Windermere if the pumping station is overwhelmed by rainfall.
Prime Minister Rishi Sunak's spokesman said that "sewage discharged into our waters like this is completely unacceptable and water companies need to clean up their acts", adding that the Environment Agency was investigating.
"If they find a breach has taken place they will not hesitate to take enforcement action up to and including criminal prosecution," he added.
The Liberal Democrat party's environment spokesperson Tim Farron, whose constituency includes Windermere, called the incident "a scandal".
"The public are rightly furious that their favourite local rivers and lakes are being spoiled while water company bosses accept huge bonuses," he said.
Windermere has suffered from algal blooms in recent summers, which has been blamed partly on treated and untreated sewage.
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