The authority’s environment spokesman says the council has been compelled to keep the road closed until May 18 - it was originally expected to reopen by around Wednesday next week.
Councillor Paul Garner said: “We have had no choice but to extend the closure of the A801 Avon Gorge to allow emergency repairs to be carried out on this strategic route.
“We apologise for any inconvenience and can assure users of this road that it will be reopened as soon as it is safe to do so.”
Emergency repairs started on March 18 but unforeseen difficulties - including sourcing specialist materials and installing additional drainage measures - have forced the council to extend the closure period.
The busy route, which links Falkirk and Bathgate, was closed on Wednesday, March 21 for emergency repairs after significant cracks were discovered in the carriageway.
The delay means drivers will continue to face extended journey times as they travel the eight mile diversion route.