The Rest and Be Thankful road was expected to open today after as much as 50 tonnes of rock and earth shifted down the hillside beside the route.
The landslide did not reach the A83 itself but the carriageway was closed on Wednesday to allow an inspection to be carried out.
Transport Scotland said the road would not open until this morning, meaning drivers faced a 26-mile detour. Northbound traffic had been directed via the A82 at Tarbet on to the A85 and Tyndrum and then the A819 Dalmally to Inverary. The diversion had worked in reverse for south-bound vehicles.
TranServ engineers were on site from early yesterday morning and were expected to continue monitoring the site this morning before making a decision on reopening the road to traffic.
A Transport Scotland spokesman said: "Following a period of very heavy rainfall TranServ engineers have reported that a slip has occurred further up the hillside and that approximately 30 to 50 tonnes of material has slipped, but not reached the road.
"We are working with Argyll and Bute Council, the police and other parties to ensure a co-ordinated response to this incident."
It is the fourth time the Rest and Be Thankful has been closed by a landslide in the past five years. A total of £16 million has already been spent on improving the road and a further £1m has been pledged.
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