There are 21 bridges in the county in poor condition and categorised as substandard - unable to bear the weight of the heaviest vehicles on the road.
Transport experts say widespread improvement of roads and bridges is desperately needed but severely underfunded.
Repairing all of Northamptonshire’s 1,203 bridges would cost an estimated £31 million, according to analysis of 2017-18 data by the RAC Foundation.
Bridges could be substandard because they were built to earlier design standards, or they may have deteriorated through age and use.
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Many of weaker bridges will be subject to weight restrictions, or written off altogether if the local authority decides decides they aren’t repairing it.
Northamptonshire County Council said that if it had no budget restrictions, it would like to bring 18 of these bridges back up to full carrying capacity.
But budget constraints mean it anticipates only one bridge having the necessary work done within the next five years.
The analysis was carried out in partnership with the National Bridges Group of ADEPT, a group representing local authority leaders.
RAC Foundation director Steve Gooding described the findings as “worrying”.
He said: “Establishing the condition of our highway bridges provides a litmus test for the condition of our road network.
“The headline message is that councils nationally are facing severe underfunding for all aspects of road maintenance, not just to fill in potholes.
“We should all be concerned when bridges along major routes are not able to carry the heaviest vehicles on the road. Many thousands are subject to enhanced monitoring, speed and weight restrictions, and the cost of bringing them up to scratch is continuing to mount.
“Longer term, the growing maintenance backlog risks pushing more and more bridges into the most worrisome category.”
The RAC Foundation estimated that it would cost £6.7 billion to clear the backlog of repairs for Britain’s bridges.
Local Government Association transport expert Martin Tett said the study “underlines the chronic need for more investment in existing local roads”.
He said: “While the extra one-off £420 million funding announced in the Budget will help, only long-term, consistent and fairer government investment in local road maintenance can allow councils to embark on the widespread improvement of our roads and bridges that is desperately needed.”