The Green Party is backing the growing No campaign in the referendum.
"Once a Political Mayor wins power they can't be removed for four years" warns Green Party spokesperson Jenny Jones. "Now matter how incompetent or arrogant they are the Political Mayor's position is secure.
"In fact in America where they've had Mayors for some time they have terrible problems with corruption and cronyism. Over 50 Poilitical Mayors from the USA are currently in prison for corruption.
"At the moment Southwark Council has to hold its meetings in public. But a Political Mayor could refuse to listen to local people. In fact they could make all their decisions in secret shutting out local people."
All Southwark's Liberal and Conservative councillors and most Labour members are opposed to an elected executive mayor.
Residents vote on Thursday 31 January.