The road used to divert traffic past a landslide at the Rest and Be Thankful in Argyll is being closed overnight as a precaution.
a “significant landslide” at the same location for the second time in six weeks.
Work to clear 5,000 tonnes of debris is ongoing.
The new closure follows further rain in the area.
It means drivers will have to follow a 59-mile (95km) diversion.
Safety assessments will take place from 07:00 on Wednesday to determine if it is safe to reopen the Old Military Road.
Eddie Ross, of Bear Scotland, said: “The standard diversion route via the A83, A82, A85 and A819 will be reinstated from 7.30pm tonight.
“The clear-up operation is progressing well which is allowing us to gauge the damage caused to the road, drainage and safety barriers.
“Safety has to come first and as ever we thank the local community and road users for their patience while we address the situation at the Rest.”
He added: “Argyll remains very much open for business and we ask road users to plan their journey in advance by checking Traffic Scotland for the real-time travel information.”