305 bridges across Wales deemed substandard by RAC Foundation
There are 305 council-maintained bridges in Wales that are substandard and not fit to carry the heaviest vehicles, according to figures from the RAC Foundation.
It said work required to bring them up to a good standard would cost £100m.
A total of 4% of Wales's council-maintained bridges were deemed unfit to carry the biggest HGVs "routinely seen on the country's roads".
A Welsh Local Government Association spokesman said work was "prioritised".
The council with the highest proportion of bridges deemed substandard was Conwy, with 51 (22%) of its 234 bridges needing work totalling £2m.
The council with the most was Powys with 62 (5%) out of its 1,336 bridges deemed substandard and Carmarthenshire with 55 (7%) out of 798.
According to the figures, Denbighshire needed the biggest outlay, with £14m required for 20 (7%) of its 280 bridges deemed substandard.
The RAC Foundation analysed data from about 72,000 bridges on the local road networks across the UK.
It said 3,203 structures over 1.5m in span were substandard which meant they could not carry the heaviest 44 tonne lorries and had weight restrictions in place or, in some instances, were closed completely.
However, there were varying reasons for them being classified "substandard". For example, some were on country tracks where heavy vehicles would not venture, so there was little incentive to strengthen them.
Percentage-wise, Conwy had the ninth highest number of substandard bridges of any British local authority.
However, those above it included four London boroughs that have between four and 11 bridges each and Blackpool, which has 21.
In Wales, Conwy was followed by Cardiff (with 102 structures) and Merthyr Tydfil (37) which both have 14% of their bridges in classes as substandard.
Both Denbighshire (280 bridges) and Carmarthenshire (798 bridges) have 7% that are substandard.
Figures for Flintshire council were not included.
The research found the cost of bringing all across the UK up to perfect condition would be £890m, or £278,000 per structure.
A WLGA spokesman said councils regularly monitor their networks, including bridges.
"Given the financial pressures faced by local authorities it is necessary to prioritise maintenance expenditure," he added.
"Should a serious issue be identified with any structure then local authorities would take necessary remedial action, or even prohibit use in a worst case scenario."
Conwy council said it had been reviewing and strengthening its road bridges and the current number deemed substandard was 46 - 16%.
It added that listed ancient monuments such as Pont Fawr in Llanrwst were included in the data and such structures are deemed acceptable in terms of structural capacity, but are unlikely to achieve the full loading capacity standard, due to their listed status.