Squatters were feared to have been trapped in an "inferno" that broke out in a vacant pub in South Harrow this morning (Friday).
The blaze has destroyed the inside of The Matrix in Alexandra Avenue that closed several months ago, and the junction of Eastcote Lane and Alexandra Avenue remains closed off.
Firefighters faced "very dangerous" conditions in the two-storey venue, which may now be structurally unsafe and could even collapse.
Eight fire engines and three aerial platforms - at least 40 firefighters - are still tackling the fire, which is expected to continue to burn until lunchtime.
Geoff Brace, white watch manager at Northolt fire station, was first on the scene with his colleagues.
He said: "The only thing that was visible were flames coming out of an external toilet extractor fan and there was a bit of very light smoke. The fan was approximately 9ft high on the wall of the banqueting suite.
"We gained access by breaking the hoardings and the doors down and I sent crews in there with firefighting jets to fight the fire and to search for people that way or may not be in there.
"We had reports that people were living in there and the building had been completely boarded up."
However, no-one was found to be occupying the former pub.
Mr Brace said: "It was completely full of smoke. We located the fire, which had progressed.
"I can say fairly confidentially it had been burning for quite some time beforehand.
"It had been burning with very limited oxygen. It was very arduous conditions in there - the crews came out feeling very hot.
"Where the seat of the fire was, I still don't know, but the bit we found was on the ground floor halfway up a flight of stairs. It's an inferno."
The watch manager, who spoke to the Harrow Observer having returned to Northolt fire station, said: "It's very dangerous in there. The floors are burned through, the ceiling burned through and the roof has burned.
"You have got large chimney stacks that are unsupported and could fall at any time.
"It's a large fire, a significant fire for us in this area. They're still fighting the fire, sprinkling lots and lots of water over it: it's a very deep-seated fire."
The emergency services were alerted to the blaze by a man who was passing by and spotted the flames coming out of the fan.

Traffic has been diverted to King's Road and Petts Hill is closed, access only.

Picture taken by Christopher J Ingram, cjphotos.co.uk .