Nine in ten bridges on Scotland’s main roads have defects which need repairs, transport secretary Michael Matheson has admitted.
The bill has also rocketed by more than 50 per cent to £570 million over the last eight years.
Scottish Labour, which requested the figures, said the situation “will leave many members of the public worried”.
A total of 1,720 of 1,933 bridges on the trunk road network need work - or 89 per cent.
Mr Matheson also said he was unable to say when the backlog for would be cleared on the bridges, which are run by the Scottish Government’s Transport Scotland agency.
The revelation comes weeks after Scotland on Sunday highlighted that its chief bridge engineer had called for a major increase in funding “to prevent issues becoming critical”.
Hazel McDonald said: “We have a backlog of work which needs to be addressed.”
She urged the current £64m annual budget be increased towards £100m over the next few years.
It fell from £65m last year.
The public spending watchdog Audit Scotland has reported the repair backlog was £60m in 2004 and £372m in 2010.
Mr Matheson said: "We are unable to determine an accurate estimate for the time to clear the current bridge maintenance backlog because the calculation is dependent on a number of unknown variables, such as future funding levels, deterioration rates and network availability.
"We will continue to use well-developed asset management practices to ensure the areas of most need are prioritised to keep the trunk road network safe and available for use."
Labour has pledged to “rebuild Scotland with a transformational £70 billion over a decade”.
Transport spokesperson Colin Smyth MSP said: “Scotland’s infrastructure has been left to rust after 11 years of SNP mismanagement and Tory austerity.
“Almost nine out of ten bridges require repairs under Transport Scotland’s watch, totalling a backlog bill of more than half a billion pounds.
“To make matters worse, Michael Matheson doesn’t have a clue when the repairs will be completed.
“The [Scottish] government’s own transport agency have made clear they are being underfunded and the current annual budget falls way short of the money needed to stop the situation becoming critical.
"That will leave many members of the public worried.
“It is important the transport secretary now clarifies just how serious these defects are and listens to the warnings over a lack of investment.”
A Transport Scotland spokesman said: “There are no bridges on the Scottish trunk road network out of use as a result of requiring repairs.
"There is a strict inspection and safety regime for all our bridges, which is based on best practice and national guidelines.
"We also have maintenance strategies for all major asset types, including bridges, in order to make best use of available funds and ensure they remain safe and fit for purpose.”
The spokesman said the overall road maintenance budget had been increased.
He said: “The safety of drivers using our roads is our top priority and that is why the budget for maintenance of the trunk road network, including bridge maintenance, has increased by around £65 million to £433 million in this financial year."
He added that the repair backlog had grown as more bridges had become the responsibility of Transport Scotland, such as the Forth Road Bridge in 2015 and the Queensferry Crossing last year.