A build-up of crumbs was enough to set alight a toaster belonging to students in Chelsea in a high-rise block.
It happened at a halls of residence in Manresa Road and resulted in around 200 people being evacuated from the building.
Students, on the 10th floor, had initially placed a fire blanket over the toaster when they discovered it ablaze, and unplugged it from the mains.
But when it continued to burn they poured water over it, before the smoke alarm sounded and fire crew were called.
Fire investigators believe the incident started after the toaster was used repeatedly in a short space of time on the highest setting, continually toasting the crumbs.
A London Fire Brigade spokesperson said: “We know toast is a staple in many students’ diets but this incident shows why it’s important to regularly clean out your toasters.
“It’s extremely lucky that nobody was injured in the fire, in particular the students who attempted to put out the blaze themselves. You should never try to fight the fire yourself.
“The most important thing to remember if a fire starts is to get out, stay out and call 999.”
The incident happened at 9pm on March 8. One fire engine from Chelsea attended but the fire was out on arrival.