A Thames Water spokesperson said misconnected pipes had a serious impact on the environment.
“Most misconnections will have been done entirely by accident but is the responsibility of the property owner, which is why we would urge freeholders or developers fitting new connections to make sure they’re plumbed in properly,” they said.
“We have a programme of work to help identify and investigate misconnections and we also fund local projects across our region.”
Mr Morris said builders and building inspectors should be liable “for errors they make or fail to detect”.
He added that councils should appoint specialists to enforce any rectifications.
A spokesperson for London Councils, the organisation that represents the capital’s boroughs, said misconnections were a “longstanding problem”.
“Preventing misconnections relies on a range of actions – and compliance with building regulations is an important part of this,” they said.
“London boroughs are under considerable resource pressure but continue to work with partner agencies and their communities to tackle this issue, including taking enforcement action where necessary.”