TTC customers heading downtown this weekend should be aware of a subway system shutdown to upgrade the signalling system.

The will be no subway service along the entire "downtown U” of the subway system between St. George and Bloor-Yonge stations.

In place of subway trains, the TTC will operate shuttle buses between St. George, St. Andrew, King and Bloor-Yonge stations.

TTC spokesman Brad Ross said the complex nature of the work makes the weekend-long shutdown necessary.

“We have a signal system that is 60 years old … and it needs to be replaced," he said. "So we’ve been doing this work in phases over the last couple of years. It’s complicated, complex work that requires more than just the usual overnight maintenance window. We need three days of testing.”

Wheel-Trans and Community Bus service will be available for passengers with mobility issues.

Regular subway service resumes on Tuesday.