Around 100 firefighters and 15 fire engines spent hours battling a blaze in Barking, east London.
The fire, which engulfed the first to sixth floors of a block of flats at De Pass Gardens, forced an evacuation of the building.
Images on social media showed flames shooting out from the wooden cladding on the front of the building.
Roads were closed off to the public as emergency services battled to put out the fire.
Firefighters were called at 3.31pm and finally got the fire under control at around 6.30pm. Fire crews from Barking, Dagenham, East Ham and surrounding fire stations attended the incident.
The cause of the is not known at this stage although rumours on social media speculated that a barbecue was being used at the time of the blaze.
Huge plumes of smoke could be seen billowing in the sky as the ground floor to the sixth floor were ravaged by the fire.
An investigation has now been launched after the fire caused extensive damage to the block of flats.
Twenty flats were destroyed by fire and a further 10 were damaged by heat and smoke.
One man and one woman were treated at the scene after suffering from smoke inhalation. No other injuries were reported.
A spokesman for the L&Q Group, which oversaw the regeneration project in east London, said: "This property was built by a private company and is now owned by a private landlord. As we have a large presence in the area, we have arranged for staff to open up our local office so we can assist residents."
