The GMB Southern region which is highlighting opposition to the Nine Elms Bridge plan has called on Mayor of London Sadiq Khan to withhold support.
The cycle and foot crossing would cost around £100m according to opponents.
The union statement follows a written reply by Sadiq Khan to a question in the London Assembly.
The unpopular proposed bridge would be opposite the already controversial new US Embassy at Nine Elms.
One issue is how much attractive open water should be covered by new crossings. This was a major objection to the Garden Bridge at Waterloo which failed to win local support.
The GMB claims that the following now oppose the major landscape change at Nine Elms:
Leonie Cooper, London Assembly member for Wandsworth
Tony Devenish, London Assembly member for Westminster
Sian Berry, Green leader London Assembly
Caroline Pidgeon, leader of Lib Dems of London Assembly
Westminster Labour Group
Wandsworth Labour Group
Westminster Conservative Party
Battersea Labour Party
North Battersea Alliance against the Bridge
Marsha de Cordova, MP for Battersea
Rosena Alin Khan, MP for Tooting
Ibrahim Dogus, former parliamentary candidate for Westminster South
Mark Field, MP for Westminster South