Emergency services were called late on Friday evening to a single-vehicle collision near to Queen Alexandra hospital on London Road
Firefighters from Cosham made the vehicle safe and used specialist environmental packs to stop leaking fuel from entering the watercourse.
The road was blocked temporarily prior to the arrival of officers from Hampshire Constabulary.
A specialist clear-up operation and recovery of the vehicle are now underway.
Specialist lighting contractors have also been called to make safe the damaged electrics from the lamp post.
It’s understood that the driver of the black Mercedes lost control before demolishing road furniture.
The driver of the Mercedes was taken to Queen Alexandra Hospital to be checked over.