The Department for Environment, Food and Rural Affairs said the 30km long tunnel will be the largest project of its kind ever undertaken in the UK.

The announcement follows six years of detailed work by a range of specialists. Defra has received reports by the Thames Tideway Strategic Study, Jacobs Babtie and Thames Water. All recommended a major tunnel under the Thames.

Until now, discharging excess sewage into the Thames during storms has been the only way of preventing an overloaded system from backing up and flooding homes. Sewage diluted with storm rainfall is discharged into the River Thames more than once a week on average, which equates to approximately 32 million cubic metres a year.

Defra expects the project to build the tunnel to be delivered between 2019 and 2020. Ofwat will ensure the project is delivered cost effectively.

Work will now go ahead on further detailed design, planning and funding.

Defra estimates that the project will add approximately £37 to Thames Water sewerage bills by 2017.
The Government last year shortlisted two options to deal with the sewage overflow: A single tunnel over 30km long under the length of the Thames, or two shorter tunnels, one in the east and one in the west of the city.

Defra said the single tunnel and side link to Abbey Mills Pumping Station was chosen because it provides the opportunity for phased construction, which will deliver results more quickly and will help meet the requirements of the Urban Waste Water Treatment Directive.

Sewage diluted with storm rainfall is discharged into the River Thames more than once a week on average, which equates to approximately 32 million cubic metres a year.