Damage was caused to a bridge by an unidentified vehicle

One lane of the dual carriageway was closed following damage to a crossing near the Castlecarey viaduct.

Central Scotland Police urged motorists to avoid the area and at one point estimated delays of up to two hours.

A stretch of the northbound lane was being closed overnight from 2130GMT for repairs. Both lanes were expected to be open for Thursday's rush-hour.

A six-foot chunk of the Haggs bridge, which goes over the road, was damaged during the night - possibly by a crane on an as yet unidentified lorry.

Engineers said the structure, just north of the viaduct, was unsafe and a lane was sealed off while the damage was repaired.

Diversions operate

Police at one point estimated a nine-mile tailback going north, with vehicles trying to join the A80 from the M73 being forced to queue.

On Wednesday evening drivers were being urged to avoid the area if at all possible.

It is expected that diversions will operate during the next three nights while repairs are carried out.

Traffic will be diverted away from the A80 at Old Inns, Cumbernauld, before rejoining the road at Haggs.