Thousands of council-maintained road bridges are substandard and not fit to bear the weight of the heaviest lorries, a study has found.

Some 3,203 bridges in Britain are not fit to support the largest HGVs, which can weigh up to 44 tonnes, according to motoring research charity the RAC Foundation.

Many of these structures have weight restrictions and others are under programmes of increased monitoring or managed decline.

The number of substandard bridges has leapt by more than thirty five per cent over the past two years, the analysis revealed, to around 4.4 per cent of the 72,000 bridges on the local road network.

Some of these bridges are substandard because they were built to earlier design specifications, while others have deteriorated with age and use.

RAC Foundation director Steve Gooding said: "It's the pothole backlog that normally hits the headlines but it is easy to forget all the other aspects of road maintenance that councils are involved in - from clearing ditches to cutting verges to maintaining bridges.

"In the face of growing traffic volumes and ageing infrastructure the danger is that without an adequate long-term funding settlement we will see more rather than fewer bridges with weight restrictions, with the backlog bill getting bigger all the time."

There has been a spate of lorries causing problems on bridges where they exceed the weight limit.

A historic bridge in Marlow, Buckinghamshire, was closed for months after it was driven over by a lorry more than 10 times heavier than the structure's weight limit in September last year.

A lorry driver was fined in July 2016 after he ignored eight warning signs before driving a lorry more than six times the weight limit over Swarkestone Bridge in Derbyshire, which is an ancient monument.

Devon has the highest number of substandard bridges at 2,689, followed by Somerset (1,459), Essex (1,567) and Northumberland (964).

Almost half (47 per cent) of bridges managed by Slough council are inadequate, more than anywhere else in Britain.

If funds were available then councils would want to bring 2,110 of the bridges back up to standard, but budget restrictions mean only 416 are expected to be restored within the next five years, according to the report.

The cost of clearing the backlog of work on all bridges is estimated to be £3.9 billion, but councils are currently spending just an eighth of that per year maintaining their bridge stock.

Many of the 199 councils who provided data for the study said funding and skill shortages were the biggest challenges they expected to face in maintaining the structures over the next decade.