The Banchory to Fettercairn road has been closed for four weeks after an articulated lorry collided with a bridge.
Engineers have been out to the Spittal Burn Bridge to assess the damage, after the vehicle was left dangling over the edge following the collision on the B974 Cairn O’ Mount road.
The structure has regularly been damaged by vehicles, including just a month ago.
Combined with the damage to the parapet in this latest incident, Aberdeenshire Council says the bridge is now unsafe and it will have to undertake immediate emergency repairs before the road can reopen.
The diversion will be via the A957 Slug Road between Stonehaven and Crathes.
Structures manager Donald MacPherson said: “Sadly, this is not a rare occurrence on our network of historic bridges - we have a number which are regularly damaged by vehicles, leading to costly repairs, not all of which are recovered from insurers as the vehicles responsible are not always known.
“The need to close this route for such a significant time, clearly an inconvenience to locals and visitors to the area, highlights how important some of our bridges are for the connectivity of the area.”
He added: “The drivers of large and heavy vehicles should be aware of the potential for damage to our historic bridges and choose their routes accordingly.
“In recent months we’ve been aware of large vehicles carrying loads such as static caravans and even a military tank over the Spittal Burn Bridge.”
Substantial repairs were carried out on the bridge in 2004, at a cost of £100,000, to increase the load carrying capacity and widen the approaches to it from both sides.