Wet conditions have reaggravated a landslip on the West Tamar Highway.
The southbound left-hand lane of the highway will be closed at the Tailrace area, possibly for some weeks, said West Tamar Council Infrastructure Services Manager Ian Howard.
Severe morning peak-hour delays are expected.
Motorists are advised to allow extra time or avoid peak-hour travel.
The Department of Infrastructure, Energy and Resources will open an emergency second lane to start operating on Friday afternoon.
The West Tamar Council has been working on repairing the landslip in the area for some time.