Lancashire County Council is alerting drivers to a diversion in place in the north of the county due to the closure of a bridge between Gressingham and Hornby.
The Loyn Bridge carries Gressingham Road over the River Lune, providing the only vehicle access over the river for some distance in either direction.
The 16th Century structure has been damaged after being struck by vehicles over recent days, with stonework atop the parapet wall assessed as being unstable.
Traffic is being diverted to the A65 at Kirby Lonsdale to cross the River Lune, using the A683 and B6254.
David Hurford, Lancashire County Council bridges manager, said: “We’re very sorry for the inconvenience caused by the closure of this bridge, particularly for people who live nearby and use it regularly to make local journeys.
“However, some of the stonework is currently in an unstable condition after the bridge has been struck by vehicles more than once in recent days, and we must close it for safety.
“Our only option is to close the bridge as the parapet wall has been struck on both sides which means there is not enough room to place safety barriers and allow room for vehicles to pass.
“Despite the inconvenience caused by the long diversion I would ask people not to remove the safety barriers, both for their own safety and anyone following them.
“We will let people know more about when the bridge may be reopened after we have made a full assessment of the damage.”
The Loyn Bridge was badly damaged during Storm Frank on Boxing Day 2015, being closed for several months for repairs to the bridge piers.
The bridge is a scheduled ancient monument and is Grade II* Listed.