The Victoria Ward at the Friary Hospital is to remain closed until at least June, NHS bosses say.
The ward at the Richmond hospital was shut to new patients in August last year because of problems with the water supply.
South Tees Hospitals NHS Foundation Trust, which runs the hospital, says work on the supply has now finished.
The trust said in a statement: “NHS Property Services is now working with the building’s landlord to complete the additional work that needs to take place elsewhere in the building before the ward can again re-open to inpatients.
“NHS Property Services has informed the trust that it currently expects this additional work to finish in June.
“Our clinical teams are looking forward to welcoming patients to the Victoria Ward when this work elsewhere in the building has been completed.”