The Thames Tideway Tunnel will be a new "super sewer", the width of three London buses, which will run underground from west to east London.
It will be around 20 miles long - and will broadly follow the route of the River Thames, connecting up to 34 sewer overflows.
Sewage, which would otherwise spill into the River Thames, will then be captured in the new tunnel and transferred to a waste water plant in Beckton to be treated.
More top news
A protest is planned 6pm today outside Stoke Newington police station
Travellers left behind an estimated 250 tonnes in Bromley
Several South West Trains services have been cancelled or delayed by up to 90 minutes