New fears have been sparked over the state of Scotland's roads after the maintenance backlog on council bridges doubled in just two years.

New figures show that the total “workbank” on around 14,000 local authority bridges has soared past half a billion pounds.

This is part of a £6.7 billion UK-wide bill that grows with every year as austerity forces councils in to tough spending decisions.

The new numbers were described as a “worrying” sign of the wider state of Britain’s roads by the motoring charity which compiled them, the RAC Foundation.

The repair backlog for council bridges in Scotland rose from £252m in 2015-16 to £374m in 2016-17 and £551m in 2017-18.

These come on top of an estimated maintenance backlog bill of nearly £1 billion on bridges on Scotland’s trunk roads, although there are no comparable figures to see if that number is rising.

RAC Foundation director Steve Gooding said establishing the condition of highway bridges is a “litmus test for the condition of our road network” and described the findings as “worrying”.

He added: “While we should draw some comfort from the good knowledge highway authorities have about the strength and structural integrity of their bridges, the fact is that many thousands are subject to enhanced monitoring, speed and weight restrictions, and the cost of bringing them up to scratch is continuing to mount.”

Bridge maintenance has hit the headlines ever since a major 1960s motorway flyover collapsed in Genoa last year. The Morandi Bridge fell in to a river in August, killing 43 people and crippling the local economy. Italy has been engulfed by a scandal over under-maintenance every since.

The RAC Foundation is not saying that there are similarly dangerous structures on Scottish or UK highways. But it did say the maintenance backlog included checks on post-tensioned or PT bridges using similar technology to the Morandi.

These PT bridges require intrusive inspections that can cost £100,000. Almost a third of the 605 PT bridges managed by Britain’s councils have not had this test carried out in the last 18 years, despite requiring it.

The latest study also found that some 3,177 bridges in the worst condition have been categorised as “substandard”, meaning they are unable to carry the heaviest vehicles. That includes 404 bridges in Scotland.

RAC Foundation’s Mr Gooding stressed that many of the these structures were old stone rural bridges, but weight restrictions came with costs.

He said: “Ancient bridges on rural back roads might not be the highest priority for repair, but the risk we run is that substandard structures on some roads result in heavier vehicles having to make lengthy detours.”

Bill Barker, secretary of SCOTS, the body that represents council road engineers, stressed that substandard bridges were not unsafe. Some 3 per cent of council bridges are subject to such weight restrictions and some 42 are scheduled to be upgraded over the next five years.

Mr Barker said: “The design life of a modern bridge is 120 years - and many Councils have “heritage” structures - many of Thomas Telford bridges, built in the early 1800s are still carrying traffic.”

He added: “The recent history of ever deeper cuts to Council funding will obviously have affected bridge maintenance as it has other infrastructure maintenance. However, each council will have had to make current decisions against a (long) history of previous decisions. Those decisions affect current condition.”

Only 38 bridges on trunk roads managed by the Scottish Government are substandard, about one per cent of the total. Some remote highways still have stone-built crossings, such as the listed 18th century Aray bridge on the A83 near Inveraray.