The A422 is closed from the recycling centre junction to New Road junction just outside Farthinghoe, in both directions, from today (Wednesday, August 8).
Due to recent rapid and significant deterioration, the bridge over Farthinghoe Stream is not capable of taking any traffic due to the structural concerns and will be closed until the end of the year.
The severe weather over the last few months, both wet and dry, has accelerated this deterioration, a Northamptonshire County Council spokesman said.
The diversion route will be using the A43 and the M40 between Brackley and Banbury, while access to local residents will be provided.
The spokesman said: “Northamptonshire Highways is extremely conscious of the impact to the public and have not taken the decision to close this route lightly.
“The county council has a duty, not only to safeguard members of the public, but to manage our structures so that they do not pose an unacceptable risk to public safety.
“We appreciate how important this route is to connecting village within South Northamptonshire.
“We will be monitoring all traffic and the impact the diversion and HGVs are having on the area during this closure.
“We are now working with all partners to put in place a programme of repair works to get this route open to traffic.”
Permits are needed from utilities companies and the Environment Agency, adding to the delay before work can begin.
The spokesman added: “Every effort is being made to re-open the road as quickly as possible.
“We are currently investigating the possibility of opening the road at the end of the year, under temporary traffic lights, to allow traffic to use the route again. This will only happen if we are able to prop the bridge and make it safe.
“A full closure would still be required for the actual repair works. At this time we are unable to give any further timescales but will update, as we are able to.
“We would like to apologise for this extended closure and for any inconvenience.
“Northamptonshire Highways is committed to safety and this route must remain closed until we feel it is safe to reopen.
“All road users are advised to avoid the area or follow the signed alternative route. Please follow the official diversion which is clearly signed and shown on the Roadworks.org website.”