Comedians Steve Coogan and Lee Mack take part in a protest against sewage discharge in the Lake District.
Campaigners gathered at Windermere in Cumbria, claiming that the lake had been damaged by pollution.
Mr Coogan, best known for his role as Alan Partridge, said water utility United Utilities was the “primary culprit”.
The company says it is investing £900m to improve its network.
Zoologist Matt Staniek claims a significant amount of phosphate in the lake comes from sites owned by United Utilities, which are harboring algal blooms.
Blue-green algae blooms can cause disease in humans and kill animals, and affect fish populations.
It was previously thought that phosphates could also come from homeowners’ septic tank flushing and vacation permits, and from farmland wastewater, but this is disputed.
According to Environmental Agency data, there are 246 days in 2022 when United Utilities will discharge stormwater into the Lakes of Windermere.
Storm surge systems are large pipes integrated into a combined sewer network that automatically discharge wastewater into rivers or seas during heavy rains, to prevent waste from entering homes.
“Last year, algal blooms were indicative of high phosphorus levels,” Mr Coogan told the BBC.
“There are huge flowers, it’s quite amazing, [it was] a very fluorescent green, it can’t move.”
Mr Coogan, Mr Mack and Paul Whitehouse were among the comedians who agreed to support Mr Staniek, the protest organizer.
Mr Coogan said: “While I have a very close local relationship here, it’s a broader issue across the country and Windermere is the largest lake in the UK, the Lake District is [a] UNESCO Heritage Site.
“We’re here to tell United Utilities, simply stop dumping wastewater into Windermere – and use their resources to reverse the damage that’s been done over the past few decades.”
At Bowness-on-Windermere, campaigners held up “Save Windermere” and “Pootopia” banners, with some criticizing United Utilities.
Paul Whitehouse, who has worked on the documentary series Our Troubled Rivers, said he’d like to see a “linked” approach.
“People are appalled at the way they’re treating our water system and they’re going to have to change, they’re going to have to move and change policy,” he said.
“The ball is in their field – it’s not going away, I’m optimistic about the future.”
‘Solve this issue now’
United Utilities said it was aware of “concerns” and was committed to “playing its part” in reducing its impact on the aquatic environment.
The company, which has previously said it has taken measures to reduce phosphate contributions from its systems, said it was accelerating a multimillion-pound improvement program.
Helen Apps, from the company, said: “We’ve put together a plan to really start tackling this now.
“We have announced that we will fast forward a £900 million investment over the next two years to commence a major overhaul of the region’s wastewater network.”
However, she said it has faced challenges including rising temperatures, climate change and increased tourism.
“That’s why we all have to work together as a community to make sure that we’re treating wastewater from the systems that United Utilities manages, we’re reducing flooding,” she said. storm flooding and we’re also looking at what can be done to improve septic tank waste and also agricultural waste – it’s going to be a joint project.”