Traffic was snarled on one of the Greater Toronto Area's busiest highways Thursday morning following an overnight rollover of a taxi cab.

All but one of the southbound lanes of Highway 400, a major north-south highway, were closed at Highway 7 after the taxi crashed into a guardrail before rolling into a ditch.

According to police, a Minute Taxi was southbound on Hwy. 400 near Hwy. 7 shortly before 4:00 a.m. when the driver lost control of the vehicle, swerved into the left guard rail before veering across all lanes and narrowly missing a light pole. It then rolled into a ditch.

OPP Sgt. Dave Woodford said the driver was taken to Sunnybrook hospital in critical condition with head injuries.

The OPP's accident reconstruction team is investigating.