The A83 Tarbet to Campbelltown road will remain closed overnight at the Rest and Be Thankful due forecasted heavy rain.
Specialist engineers will assess the safety of the route on Thursday morning before determining if it can reopen.
As a result, the Old Military Road diversion will remain in use overnight.
The diversion, a private single-track road which runs parallel to the A83 through Glen Croe, was implemented at around 8am this morning with traffic being convoyed through the area.
The A83 is expected to reopen tomorrow mid-morning under traffic light control following a full safety assessment.
Teams are continuing to carry out mitigation work on the hillside to provide further protection from landslips.
Eddie Ross, Bear Scotland’s north west representative said: “We’ve been closely monitoring the weather forecast as well as conditions on site throughout the day and we’ve taken the decision to keep the Old Military Road in use overnight for road users as a safety precaution.
“Conditions are set to improve overnight so we’re targeting reopening the A83 under traffic light control mid-morning tomorrow following a full safety and geotechnical inspection.
“We need to ensure safety of road users is prioritised at all times, and as ever we appreciate and thank the local community and all road users for their patience.
“Argyll remains for business and we’ll continue to do everything we can to keep any disruption to a minimum.”
The closure of the route comes after substantial landslips in recent weeks which brought tonnes of debris onto the highway.
Calls have been made for a permanent solution to prevent the vital route from being closed in the future.