Plans are under way for a new giant tunnel to transport sewage away from Windermere, England’s largest lake.

Windermere, in the Lake District, has suffered from widespread raw sewage and bacterial pollution driven by discharges from United Utilities. Campaigners say this pollution has caused more frequent toxic blue-green algal blooms, with the public advised to avoid swimming in the lake.

Modelling indicates the climate crisis will exacerbate these challenges, with the catchment expected to experience more intense rainfall over the coming years, placing further pressure on the wastewater infrastructure.

A new project, backed by United Utilities, the Environment Agency and local campaigners, has set out a vision to end sewage discharges into the lake by building an 12-mile (19km) tunnel to transport sewage away from the lake.

The water company said the use of tunnelling techniques would minimise the disruption to local communities.

Windermere is at the heart of the Lake District Unesco world heritage site – it has been the inspiration for poets, storytellers and lyricists for generations, with an estimated 17.7 million people visiting every year.

The treated effluent would then be discharged into either the River Leven or the Irish Sea at Ulverston. It is estimated the project will cost £3.6bn, with United Utilities to seek permission from the government to raise customer bills to pay for it. It said the programme could add up to £3.17 per month to water bills in 2035.

The project will now enter the detailed design phase during which specialised engineers will work out the technicalities of the build, which is expected to take two years.

If granted permission to raise bills to pay for the project, United Utilities said the project could be complete within seven years and would provide a “one and done” solution with an immediate impact.

Matt Staniek, a campaigner at Save Windermere, which is a partner on the project, said creating the feasibility report had involved weekly meetings over 15 months.

Staniek, who has been calling for the end of sewage discharges into the lake for more than five years, said he was concerned at first about partnering with the company causing the pollution, but said they were the only people who could actually fix it.

“This has demonstrated that it is possible not to dump sewage in Lake Windermere,” he said.

In planning for the project, the team considered the transformation of Lake Annecy in France, which is widely recognised as one of the cleanest lakes in Europe.

In the 1950s, Annecy faced similar pollution problems, but the authorities invested in long-term infrastructural improvements to eliminate the sewage discharges.

“This is up to the government now. The plan is there; it’s in place, we’re not going to stop until Andy Burnham is on the shores of Lake Windermere with a spade announcing that the construction has begun on this project”.