The fire investigation was launched after 80 firefighters tackled a fire in the roof of an exclusive apartment building in Belgravia.
Twelve fire engines were called to the fire at Eaton Place in Westminster at 11:40 a.m. Thursday.
Houses on the exclusive street can fetch up to £20 million on the market.
The street is also home to the Hungarian Embassy.
Clapham, Dowgate, Soho, Hammersmith and other surrounding fire stations were on the scene until just before 5pm.
The roof was completely destroyed, according to the fire service, while extensive damage was done to a seven-storey apartment on the fifth floor. Flats on the third and fourth floors were also hit, but no injuries were reported.
A spokesman for the London Fire Brigade said: “A significant part of the roof and one flat were on fire.
“The brigade control office had received 26 calls about the fire.
“The cause of the fire is unknown at this time.”
Theo Coen, 18, who lives in the road, said he woke up at 11:30 am to the sound of “cracking”.
He added: “My mother says there is a fire and we leave the place and we just see the fire going around. It won’t be long before the police, the fire brigade, ambulances arrive.”