Commuters face major delays after Bank Bridge on the A59 near Tarleton was badly damaged when it was struck by a vehicle over the weekend.
The incident has left a gap in the parapet wall on the Preston bound side of the bridge.
For safety the road is currently being restricted to one-way traffic controlled by temporary lights while Lancashire County Council engineers carry out urgent repairs.
Staff have been working over the weekend to recover the fallen stonework so that the damaged wall can be reconstructed as soon as possible.
As the bridge is a Grade II listed structure, much of the original stonework will need to be reused.
Engineers are now working to complete the job as quickly as possible, with the bridge due to be fully reopened by Friday (November 14).
Drivers are asked to consider using other routes if possible to avoid delays, particularly during peak times.