The leaflets say that the new sewer - the Thames Tideway Tunnel - is needed to help clean up the River Thames.
Tens of millions of tonnes of untreated sewage currently overflow from London's Victorian sewers after as little as 2mm of rain.
Water bills will have to go up, they add, to help raise the £4.1 billion needed to complete the project.
Thames Water has announced that bills could go up by as much as £80 a year to help pay for a major new sewer.
Leaflets are being distributed to customers to warn them that they'll be charged more from 2014, in order to raise money for the Thames Tideway Tunnel.