A make-up mirror focusing the sunrays is believed to have caused a flat fire above a pub on Collier Row Road in Romford.
The Brigade was called by a passer-by who observed a crowd outside and looked up to see smoke issuing and flames at a window on the first floor.
Charlie Pugsley from the Brigade's fire investigation unit said:
"These sort of fires are not as rare as you would think. I’ve seen everything from sparkly door knobs to crystal balls starting fires.
"Crystal and glass ornaments and items such as mirror tables should be kept out of direct sunlight. This shows these fires are not an urban myth. Also make sure that you have a working smoke alarm on every level of your home."
Part of the first floor flat was damaged by fire. Around 50 people evacuated from a pub below. There are no reports of any injuries.
The Brigade was called at 1352 and the fire was under control by 1503. Fire crews from Romford, Hainault, Harold Hill and Dagenham fire stations were at the scene. The cause of the fire is under investigation.