Four fire engines and around 25 firefighters were called to a fire at a flat on London Road in Thornton Heath.
Part of a six-roomed flat on the first floor of the building was damaged by fire. There were no reports of any injuries.
The Brigade's Fire Investigation team believe the fire was caused by a toaster which was accidentally left on.
A London Fire Brigade spokesperson said: “It’s really important that you always check toasters and other appliances are turned off and unplugged when not in use, unless they’re designed to be left on- like your fridge or freezer.
"Keep clutter to a minimum on your kitchen work surfaces especially near toasters and hobs as they can accidentally switch them on or catch fire."
The Brigade was called at 0817 and the fire was under control by 0859. Fire crews from Norbury and Croydon fire stations attended the scene.