The first two 'New Routemasters' or 'Boris buses' to be introduced on route 12 came into public service on Saturday.
The new vehicles will be phased into service on route 12 which runs between Oxford Circus and Dulwich over the coming weeks.
Until 2011, the 12 used controversial articulated 'bendy buses'.
The 12 is the third route which runs via the Elephant & Castle, Westminster Bridge Road and and Westminster Bridge to be converted to 'New Routemasters'.
Although the open platform was touted as one of the vehicle's most important features, none of the 'Boris bus' routes south of the river have a second crew member, so the doors stay closed between stops.
This December will be the tenth anniversary of the withdrawal of the last old-style open platform Routemasters to operate on daily public service in SE1.