Subway service has been suspended in both directions on Line 1 between Union and Eglinton stations, the TTC says.
Officials say a signal was malfunctioning at Bloor-Yonge Station.
TTC spokesperson Brad Ross told Newstalk 1010 that the issue related to a failsafe system that detects whether a train has departed the station or not.
The TTC tweeted that shuttle busses were dispatched to carry passengers to and from the 12 affected stations.
Affected customers can call the TTC's customer info line at 416-393-4636 for help finding an alternative route.
