The TTC admits it may have to push back the scheduled completion date of the six-stop Spadina subway extension.

TTC CEO Andy Byford said a handful of factors — everything from poor weather, to problems with contractors, to a fatal workplace accident at York University station in 2011 — has led to delays that will make meeting the scheduled fall 2016 completion date "a challenge."

"We're looking to recover [from] the delays, but right now I'd say there's a risk that the end date of 2016 will not be met," he told CBC News.

The project to add six stops north of Downsview station has had a number of problems. In August, the TTC filed a breach-of-contract lawsuit against the contractor overseeing construction of the Vaughan Metropolitan Centre station.

In October 2011, a worker was killed when a drilling rig collapsed at the construction site of York University station.

Byford admits the delays are most pressing at three of the stations: Pioneer Village, York University and Vaughan Metropolitan.

"We have for some time … been saying that we do have a number of challenges and those challenges remain and we now can say that that schedule is at serious risk," he said.

Byford said other aspects of the project are going well. Tunnelling is complete, track is being laid in some sections and the "stations are starting to look like stations."

He also said TTC officials are working with contractors to find ways to speed up construction and minimize any further delays.

"I want that railway open as soon as possible," he said.

Byford would not speculate on when the extension might be complete. He would also not speculate about how much any overruns may cost.