The closure is expected to last until at least noon, according to the OPP. Southbound traffic is being diverted off Highway 400 at King Road.
OPP Sgt. Kerry Schmidt told CBC News they believe a passing vehicle, possibly carrying a piece of over-height equipment, struck and damaged the sign.
"Crews got on the scene and looked at the structure," said Schmidt. "They assessed it and found it to be unsafe for people underneath the bridge. It would cause considerable damage if it were to come down completely.
"We are working as fast as we can to get the equipment in place and get this highway reopened."
CBC reporter Linda Ward said the sign — a massive structure that spans multiple lanes — is almost split in half and listing to one side. "A very unsafe situation," she reported.
At 6:30 a.m., Ward said traffic was already starting to back up along Highway 400. CBC traffic reporter Laurie Turcotte suggests southbound drivers use Weston Road or Jane Street and rejoin Highway 400 at Teston Road or at Major Mackenzie Drive.
Follow Turcotte on Twitter (@CBCTrafficTO) for updates.
We will also have updates on 99.1 FM.