United Utilities has insisted it takes its role in protecting the environment “very seriously” as it revealed annual earnings jumped by nearly a fifth just days after being engulfed in a “scandal” over a massive sewage spill.

The water group reported a 17.5% rise in underlying operating profits to £517.8 million in the year to March 31.

It said: “We take our environmental commitments very seriously and are proud to have a sector-leading track record on minimising pollution for over a decade.”

We take our role in protecting the environment very seriously; our ambitious business plan would see us investing more than ever before to improve services across the five counties of the North West

But on Wednesday, a fresh row broke out over the state of England’s waters after reports that millions of litres of raw sewage were pumped into Windermere in the Lake District.

Documents from United Utilities seen by the BBC showed that a fault at a pumping station in Bowness-on-Windermere in Cumbria left sewage being illegally pumped into the famous lake for 10 hours in February.

The situation was labelled a “scandal” by opposition politicians, while Downing Street said it was “completely unacceptable” and the Environment Agency had the power to launch a criminal prosecution if necessary.

United Utilities said on Wednesday that the spill was caused by an unexpected fault on the third party telecoms cable network in the area, “which United Utilities was not notified about and which affected both the primary system and United Utilities’ backup”.

It said “engineers took urgent steps to resolve the situation and we informed the Environment Agency within an hour of the pollution being confirmed”.

In its full-year results on Thursday, United Utilities said it was bringing forward around £400 million of investment in its systems to reduce spills at more than 150 storm overflows.

The group added it was moving to “accelerate environmental schemes in communities such as Windermere, where we are fast-tracking investment to drive improvements earlier”.

Chief executive Louise Beardmore said: “We take our role in protecting the environment very seriously; our ambitious business plan would see us investing more than ever before to improve services across the five counties of the North West.

“This would deliver a genuine step-change in infrastructure for the benefit of customers and the environment, and support 30,000 jobs.”