A water main break is responsible for causing a huge sinkhole on Yonge Street south of Highway 401 that is slowing traffic, Toronto Mayor John Tory says.

"This sinkhole resulted from a water main break, which led in turn to instability of the soil underneath the pipes and to a sinkhole. Now, we're focused on getting it repaired as soon as possible," Tory told reporters on Thursday.

City crews are assessing how long it will take to repair the sinkhole, located in two southbound lanes of Yonge Street at William Carson Crescent. Part of the sinkhole, north of York Mills Road, extends into the intersection.

Bill Shea, director of distribution and collection for Toronto Water, said on Thursday it will take at least until the weekend to repair the road surface. (CBC) Toronto Water said work has begun to fix the sinkhole, but no timeline has been established. The road surface has collapsed onto a four inch gas service pipe and Enbridge Gas is on the scene to repair it.

Bill Shea, director of distribution and collection for Toronto Water, said on Thursday it will take at least until the weekend to repair the road surface.

"We have a break in a service that comes off our water main, a six inch service that feeds this building. It's broken and it's undermined the roads, created a large void under the road, and the road has collapsed into that void," he said.

Shea said there have been about 250 water main breaks since Dec. 28, 2017, a number that is triple the usual amount for this time of this year.

"This increase in breaks is because of that cold snap and this now thaw. That creates a shift in the ground, and especially for our older cast iron pipes, it cracks them. The shifting of the ground is actually cracking our pipes and causing these breaks."

Toronto Water is spending on $158 million on replacing and relining water mains with plastic pipes this year, he said. The plastic pipes are not as susceptible to breaking, he added.

The sinkhole is affecting two southbound lanes of Yonge Street at William Carson Crescent, north of York Mills Road. (Mike Heenan/CBC) Transportation Services crews have taped off the sinkhole, using a series of orange pylons, to prevent vehicles from falling into it.

From above, the sinkhole looks like a large foot.

Tory says the city has decided to make water main replacement a priority and the sinkhole repair work is part of that effort.

"We are investing literally hundreds of millions of dollars in doing water main repair work, for example, and replacement work that wasn't done in previous years," he said.

"The bottom line is, in some cases, these projects had been deferred for decades. And we're now doing them. But you can't do every one at the same time. And you don't always know where the faults are."

Toronto police are suggesting that drivers in the area might want to find alternate routes.

According to Toronto Water, water mains break more frequently in winter months – November to March – because low temperatures mixed with warm spells can cause soil to freeze and expand, putting pressure on the infrastructure.

The five-year annual average of water main breaks is about 1,400 water main breaks a year. This month so far, Toronto has had 182 water main breaks.