Four fire engines and around 25 firefighters were called to reports of a garage alight on Blackfen Road in Sidcup.
Part of a garage and a small part of a neighbouring summer house was damaged by fire and a fence was destroyed by the blaze. There were no reports of any injuries.
The Brigade's 999 Control Officers took 19 calls to the fire.
The Brigade's Fire Investigators believe the fire was caused by the self-heating of straw and sawdust.
A London Fire Brigade spokesperson said: "Sometimes when materials like straw, hay or sawdust are stored in large quantities and compressed, biological or chemical reactions can occur which can result in a fire.
"These types of reactions often happen when hay is damp or damaged or when a contaminant, like oil, comes into contact with the materials.
"Make sure hay is stored in smaller amounts and protected from moisture and other contaminants."
The Brigade was called at 1234 and the fire was under control by 1321. Fire crews from Sidcup, Plumstead and Bexley fire stations attended the scene.