A diversion taking drivers away from the A83 at the Rest and Be Thankful will be used from this evening, as forecast heavy rain raises safety concerns.
From 8pm, those travelling on the route will be directed towards Old Military Road, the single-lane road that runs through Glen Croe parallel to the A83 section.
An assessment will take place at 12pm tomorrow to decide whether it is safe to reopen the ordinary route.
The decision to close the Rest and Be Thankful comes amid predictions of poor weather conditions over the next few days, with the road lying in the area of the country covered by two separate yellow weather warnings lasting from 12pm today until 6am on Thursday.
The first, for thunderstorms, will remain in effect until a minute before midnight, when a second warning for rain will come into effect to last throughout Wednesday and into the following morning.
Eddie Ross, north-west representative for road maintenance body Bear Scotland, said: “We’ve been assessing the weather forecast over the next few days and have taken the decision along with Transport Scotland to direct all traffic to use the Old Military Road from 8pm tonight as a safety precaution.
“Tonight’s forecast has indicated heavy rain throughout the night which is difficult to monitor the impact of on the hillside during the hours of darkness, with further heavy rain expected tomorrow morning.
“We’re continuing to put road user safety first and will therefore reinstate the OMR from 8pm as a precaution.”
The stretch of the A83 which connects Arrochar to Cairndow, taking in the stunning Rest and Be Thankful viewpoint, was hit by a landslip in August last year and has been closed a number of times since.
The incident was just the latest in a series of landslips that have plagued the road, which runs alongside a steep rocky slope.
Transport Scotland has proposed buiding an alternative route, but was criticised in April for launching a £25million tender for consultancy services.
In the meantime, construction work is taking place to strengthen the landslide mitigation measures, including the creation of debris catch-pits and improvements to hillside drainage.
Mr Ross added: “Teams are continuing to pay close attention to the weather conditions in the area and are monitoring the hillside for any changes.
“As ever, we thank all road users and the local community for their patience in advance while we continue with our work to address the Rest and Be Thankful.”