An early morning blaze has destroyed the structure of Black Walnut in Wortley Village, despite seven fire crews trying to put the fire out all day Sunday.

Owners, brother and sister Ed and Mandy Etheridge, stood by and watched as firefighters sprayed water on the bakery the family has owned and operated for 12 years.

About 20 employees work at the Wortey location of the cafe, said Ed Etheridge, who raced down to the scene just before 2 a.m.

"It's a devastating loss for the community," he said.

No one was inside at the time the fire started, including his father, who lives in a recently renovated apartment above the cafe, said Etheridge.

"When crews arrived on scene, they had a fully involved fire where flames were visible on the exterior and had also reached into the building," said Kirk Loveland, London Fire's platoon chief.

Loveland says his crews were at it for hours trying to extinguish the fire but were unable to because it spread so quickly.

"We kept knocking the fire down but we couldn't fully extinguish it," he said.

"It was a stubborn blaze that was continuing to work its way around the building and eventually it broke through the roof, so we had to go into a defensive strategy where crews can no longer be operating inside the structure."

Loveland estimates the damage to be at least $2 million, adding that the building will be a total loss. The cause of the fire is not yet known but the matter is being investigated, Loveland said.

When asked if he would rebuild, Etheridge responded without hesitation: "Of course."

Neighbour Morgan Allcroft was forced to leave his home in the middle of the night.

"I smelled smoke and people [were] pounding on my door," said Allcroft. "Just very surprising. My house is so full of smoke, my eyes are watering."

Crews will remain on scene for at least a few more hours, as they are in the process of knocking the structure down because it is a hazard and can collapse, Loveland said. He is advising people to stay away from the area until the fire is fully out.