Firefighters were called to Spice Klub lounge bar and restaurant on Peel Road, in Harrow at just after 6.15am this morning (Wednesday, May 25).
Dozens of firefighters fought the flames as thick grey smoke poured from the building.
The blaze was under control by just after 8.30am but the entire roof and first floor of the building have been gutted by the fire.
Part of the ground floor has also been damaged.
Nineteen people left the building before firefighters arrived.
There were no reports of any injuries.
A London Fire Brigade (LFB) spokesperson said: “Ten fire engines and around 70 firefighters were called to a fire at a restaurant.
“One of the brigade’s aerial appliances was used to put water on the fire from above.
“Fire crews are likely to be at the property throughout the morning damping down pockets of fire.
“The Brigade was called at 6.19am and the fire was under control by 8.38am.
“Fire crews from Harrow, Stanmore, Northolt, Wembley and surrounding fire stations were at the scene.
“The cause of the fire is under investigation.”