The tunnel, as first reported by CBC News Toronto, was found by a Toronto Region and Conservation Authority employee last month. It was subsequently reported to police.
Toronto police Deputy Chief Mark Saunders is scheduled to give a press conference at 10 a.m. ET to provide "detailed information," including numerous photographs.
National security officials have been notified about the tunnel, which was discovered near the Rexall Centre, Canada's national tennis facility and a venue for this summer's Pan Am Games.
Police have not made public comments about the tunnel, which is approximately seven metres long and 2½ metres high. When found, it had lights powered by a generator, and its walls and ceiling were reinforced.
Sources told CBC News that the tunnel has been filled in by authorities.
CBC News has learned that investigators visited the Tennis Canada site a few weeks ago and spoke with grounds crew at that time. While investigators were on site, there was also a police presence in the woods nearby.
No one has come forward to claim the digging tools and generator found at the tunnel or offer any explanation for its existence.
CBC News has confirmed employees at several area gas stations were asked if anyone had been filling up gas cans recently, in order to refuel the generator.
So far, no suspects have been identified.