London Fire Brigade dispatched 58 firefighters and eight fire engines in an attempt to tackle the blaze on Rosebank Road.
They said half of the workshop, filled with car tyres and tools, caught alight.
The blaze took around two hours to extinguish, after crews were called out at 5.13am.
Station manager Stuart Grout said: "Crews had to spend a lot of time cutting away at the building to access the fire.
"They made steady progress, clearing the contents of the workshop in order to reach the seat of the flames."
Witness Paul Wood said the crews' "blackened helmets tell their own story" after crews spent the morning putting out the flames.
No injuries have been reported and the cause of the fire remains unknown at this stage.