Firefighters were called to a house in Croydon when a bedroom caught fire.
The crews arrived in St Peter’s Street at about 8.50pm on Monday to tackle the blaze which damaged around half of the first floor and part of the roof.
People inside the house when the fire started managed to get themselves out and no-one was injured.
A fire investigation team believes the cause was a laptop left on a bed.
A London Fire Brigade spokesman said: “You should never leave a laptop on a soft surface such as a bed or a sofa, because the fabric can stop the computer from cooling itself properly, which can cause it to overheat and catch fire.
“Always turn laptops off when you’re finished with them, and leave them on a hard surface such as a table.”