A fire at one of Canada's premier stables and equestrian training centres has killed 43 racehorses, as firefighters battled a blaze in one of five barns.

The blaze at the Classy Lane Training Centre, about 80km east of Toronto, was discovered late on Monday and firefighters worked through the night to put it out, said Jason Benn, chief fire prevention officer at the Puslinch Fire and Rescue Service.

"It was a very intense fire. The building was fully engulfed" when firefighters arrived shortly after 11 pm, Benn said, adding that five fire services from the surrounding area fought the blaze for nearly six hours.

Owner Jamie Millier said all of the horses in the barn died as smoke pushed back would-be rescuers. The facility, built in 2003, can accommodate 222 horses in five barns, according to its website.

"We don't know yet what happened," Millier said.

"The barn burned down and we just couldn't get the horses out. A couple of people were coming back from the races and they pulled in and could see some smoke and they came running up, but as soon as they opened the door, black smoke drove them back."

Millier said it was too early to estimate the value of the horses lost in the fire or of the building, but said it would be "in the millions."

Asked if any famous racehorses died in the fire, Millier said they were all famous to their trainers.

"They felt the same about every horse," Millier said.

"Everybody is very devastated about it ... everybody feels sick."

The facility was described as the "finest training establishment in all of Canada if not North America" by The Harness Edge, a trade publication.