A man was airlifted to hospital with serious injuries and dozens of residents were evacuated after an explosion in a block of flats today.
The blast, which occurred in a top-floor flat, is said to have been caused by a gas explosion.
Emergency services were called to the scene at Drinkwater Road, south Harrow, in north London, at 8am this morning.
Thirty firefighters and six engines were sent to the scene.
A spokesman for the ambulance service said the injured man, believed to be in his thirties, was treated for burns at the scene and then taken to hospital. He is in a "serious condition".
The spokesman added: "We sent two ambulances, a fast response car, three hazardous area response team units and a duty manager."
He said it was usual to send the hazardous-area response-team units as a precautionary measure.
Harrow Council confirmed between 30 and 40 people had been evacuated from Annan Court - where the blast happened - to a community centre in Scott Crescent.
Council officers were at the scene liaising with emergency services.
A woman living on Drinkwater Road, where the building collapsed, said she heard a "big explosion" at about 8am.
Behnaaz Shah said: "I was on my way out when I heard the explosion and I came out to have a look.
"The building opposite looked like it had exploded and there was fire coming out of it."
She said the building's roof had caved in and a section of Drinkwater Road had been cordoned off.
Early reports from the London Fire Brigade suggested there had been "serious structural damage" to the building.
A total of 19 flats were occupied at the time, mainly with elderly residents, but no-one apart from the hospitalised man has sustained any injuries.
Harrow Council officers are at the scene to provide immediate welfare needs and are working to provide emergency overnight accommodation.
An emergency hotline - 020 8736 6000 - has been set up by Harrow Council.