Fire tore through a bedroom in south London when sunlight hit a mirror, igniting bed linen inside.
The blaze tore through the room in Peregrine Gardens, Croydon, at around 11.22am yesterday morning.
A woman was able to escape from the building after she was alerted by a smoke alarm before London Fire Brigade crews arrived. She was checked over by London Ambulance Service paramedics.
The LFB believe the fire was caused by a concave mirror which focused the sunrays onto bedding, leaving the first floor flat badly damaged.
It follows a fire earlier this year caused by the sun refracting through a Nutella jar in Twickenham, which caused the death of a family dog.
Brigade fire investigator Mark Ross said: "The winter sun seems to be more prevalent this year so I’d ask everyone to keep mirrors, bottles of water and crystal balls out of direct sunlight.
"Thankfully the resident was alerted to the fire by her smoke alarm that our crews fitted two years ago.
"It is essential that everyone has at least one smoke alarm on every level of their home."
