Multiple landslips have blocked the Rest and Be Thankful in both directions and its usual diversion.
The route along the A83 became covered in debris following heavy rainfall across Argyll and Bute on Tuesday.
Debris has also blocked the local Old Military Road, which has previously been mobilised to use as an alternative route.
Roadside assessments appear to show there is still movement on the hillside and it is unsafe to begin clear-up operations or carry out further investigations.
Both routes will remain closed overnight on Tuesday and safety assessments will take place at first light on Wednesday.
A diversion route has been put in place between Tarbet and Cairndow via the A83, A82, A85 and A819.
Temporary traffic lights are installed on the A82 at Inveruglas after heavy rain also impacted a retaining wall.
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