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Detailed plans for a £3.6bn tunnel to stop raw sewage polluting the River Thames have been unveiled.

The 20-mile (32km) Thames Tunnel would run from west to east London, broadly following the route of the river, collecting sewage discharge triggered by rainfall.

Concerns have been raised about the cost of the plan to water customers and the long-term environmental gain.

A public consultation on the Thames Tunnel will last for 14 weeks.

will be at a depth of about 246ft (75m) and will emerge to the east of Tower Bridge at Limehouse before going one of three ways:

Thames Water has estimated the cost of the planned tunnel could result in bill increases of about £60 to £65 per year for its customers.

The company said, on average, sewage is discharged into the Thames at least once a week due to rainfall.

Martin Baggs, Thames Water's chief executive, said: "Allowing sewage to continue to overflow into the river at the current frequency is unacceptable.

"This causes significant environmental damage - killing fish, polluting the river for those who wish to use and enjoy it and affecting the wellbeing of our capital.

"We have got a plan to tackle this problem and have already started work on the £635m four-mile Lee Tunnel in east London and our £675m upgrades at London's five major sewage works.

"The proposed Thames Tunnel is the final and most challenging piece of the overall plan.

But west London's Hammersmith and Fulham Council is against the plan.

Council leader Stephen Greenhalgh said: "We have consistently pushed for a shorter, smarter tunnel that minimises disruption to Londoners.

"The gold-plated 'super sewer' with a £3.6bn price tag threatens our parks and will drive many hard-working families into water poverty to pay for it."

Thames Water hopes to submit a planning application in 2012 with construction due to be completed in 2020.