Motorists faced detours of almost 60 miles after the landslip blocked the the road for the eighth time in five years.
The A83, which runs from Tarbert to Campbeltown in Argyll, was closed after heavy rain caused a landslip at Butterbridge, near the Rest and Be Thankful, around noon yesterday.
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Contractor Bear Scotland said an emergency diversion using the Old Military Road was not possible because of the location of the landslip.
Diversions in both directions were put in place via the A82 but still mean a 60-mile detour.
Bear Scotland said specialist engineers had examined the site and found the slope remained unstable and a large boulder needed to be removed.
The Old Military Road, which runs parallel to the main carriageway through Glen Croe, was reopened last year as an emergency diversion route to be used if the A83 was blocked.