The proposal, called "Only Rainwater into Windermere" (ORiW), aims to stop both raw sewage and treated effluent entering the lake. At present, when treatment works are overwhelmed by heavy rain, raw sewage spills into the lake. Even treated effluent contains high nutrient levels that have contributed to algal blooms turning parts of the lake green in recent summers.
United Utilities chief executive Louise Beardmore described the plan as a "100 year solution for this lake." She said it represents a departure from the UK's usual five-year investment cycles. "This is saying we need to do something differently," she told the BBC.
Activist Matt Staniek of the Save Windermere group, who has protested outside United Utilities offices, called the proposal a "milestone" but cautioned that it remains a plan on paper. "This is the beginning, and the end of this is the moment that sewage stops entering Windermere," he said.
The tunnel would be up to 12 miles (19km) long and 4m wide, with up to 86 miles (138km) of new sewage infrastructure and a single treatment works. Two route options are being considered: discharging treated effluent into the River Leven or the Irish Sea at Ulverston.
Funding options include adding up to £3.17 a month to United Utilities customers' bills, or spreading the cost across all English water bills by up to 35p a month. Nigel Wilkinson of the Love Windermere group, which represents local businesses, argues that the lake's national and international significance justifies national funding.
The Department for Environment, Food and Rural Affairs welcomed the plan but stressed that further work is needed to test alternatives. The government made a promise in March 2025 to stop all sewage entering Windermere.