Drivers in south west Shropshire will face diversions for up to three weeks after a bridge was badly damaged by an unknown vehicle.
Clun Bridge, which is a listed building, suffered heavy damage during the early hours of Thursday morning.
To date Shropshire County Council has not been able to trace any details of the vehicle involved.
Engineers say the work is bound to cause inconvenience and disruption but the structure is too narrow to keep open to traffic while workers repair the stonework.
Assess damage
The county council hope to partially reopen the bridge during the Clun Carnival on 9 August, but will not know for sure until experts have assessed the damage.
Shropshire County Council's principal bridge engineer, John Williams, said: "It is appreciated that the road closure will cause significant inconvenience and disruption to the local community and all reasonable efforts will be made to keep these to a minimum.
"Unfortunately, the extent of the damage is such that the repairs must be carried out immediately in order to safeguard the integrity of the bridge and ensure the safety of the public."
During the course of the closure, traffic will be diverted via Purslow, Leintwardine and Knighton.