The TTC says Line 1 between St George and St Andrew stations will be closed for 10 days starting on Monday.
Shuttle buses will be operating.
The closure will allow crews to repair tunnel lining, remove asbestos and power cables ,and clean stations, the transit agency said in a news release. It added that now is a good time to do this work because ridership is low.
The TTC said the shutdown is the final closure of this section of the downtown "U" of Line 1 and the completion of the work means the TTC can avoid between 75 and 100 planned weekend closures over the next few years.
"Customers who travel along the impacted part of Line 1 were surveyed during the previous two closures and the vast majority are supportive of the TTC's approach," the TTC said in the release.
TTC CEO Rick Leary said the transit agency is taking a different approach to its repair work and he believes it is making "significant" progress.
"With the completion of this series of closures, we will have accomplished 18 months of work in three weeks and saved customers travelling in this area, a two-year-long inconvenience," Leary said in the release.
The TTC said customers can expect shuttle bus service every 60 seconds or better during peak periods and consistent service throughout the day. Service levels are based on current customer volumes, the transit agency added.
There will be designated shuttle bus pick up and drop off locations outside each station at street level.
East-west service will continue on College (506 Carlton), Dundas (505), Queen (501) and King (504) streets, enabling customers to connect from Yonge Street.
Museum, Queen's Park, St Patrick and Osgoode stations will be closed.
St George and St Andrew stations will be open for fare sales. At these stations, customers can board buses and streetcars and connect to Line 2.
All other subway entrances and exits, including TTC connections to hospitals, will be closed along the stretch that is shut down.
Mayor John Tory said the city is committed to upgrading the TTC as quickly as possible.
"Getting harmful asbestos removed, tunnel linings repaired, power cables repaired and stations cleaned now will make sure our system is ready to welcome back more riders as the pandemic comes to an end," Tory said in the release.
During this closure, crews will finish removing all asbestos on the southbound platforms at St Patrick and Queen's Park stations.
The TTC said it is encouraging customers to begin their trips at any station on the Yonge side of Line 1 to avoid the area that is closed.
Customers who require Wheel-Trans services can speak to any TTC customer service staff member for help.