The truck crashed on a ramp leading to the Don Valley Parkway and the northbound Highway 404. (CBC)

Transportation officials warned that many Toronto drivers could expect delays on their commutes Thursday afternoon and Friday morning, after a fiery tractor-trailer crash forced them to close an overpass linking two of the city's main expressways.

The tractor-trailer had been travelling west in the express lanes on Highway 401 and taking the ramp to the Don Valley Parkway-Highway 404 when it went off the road as it passed under one of the overpasses just after midnight.

The truck, carrying paper products, exploded in a huge fireball when it crashed into a guardrail and then slammed into a concrete pillar.

"The driver managed to escape the vehicle and was taken to hospital with second-degree burns and is being treated for non-life-threatening injuries," said OPP Const. Dave Woodford.

"We don't know if he went too fast at this point or fell asleep."

The heat from the fire was so intense highway lights melted and chunks of concrete were baked off the bridge, exposing parts of the steel reinforcing, which in turn was damaged by the fire.

Engineers from the Ontario Ministry of Transportation examined the overpass and by late morning they had bad news for commuters.

"The collectors and this ramp will not be reopening today," said MTO spokesman Will MacKenzie. "There's too much damage in there. Our engineers have had a look at it as much as they can, it's still extremely hot and very difficult for them to get in an do a really, really close look."

When asked when the area might return to normal MacKenzie said, "I cannot speculate as to when." The damage, he said, is "severe."

"The [reinforcing] steel has been damaged.… We can't open up if there's any kind of risk at all."

The traffic problem was starting where westbound traffic exits the collector lanes onto the Don Valley Parkway-Highway 404. Those lanes were completely blocked Thursday by late morning and were to remain closed. Drivers were being forced to take different routes to avoid the accident scene.

The 401 westbound express lanes remained open, as did most of the collector lanes.