REPAIRS to a medieval river crossing, which serves as a city centre thoroughfare, have been prolonged as a result of flawed Victorian engineering.

Routine maintenance work began on Durham's Framwelgate and Elvet bridges in January, in what was expected to be a 12-week contract.

Scheduled monument consent was secured by Durham County Council to allow the repairs to be carried out, overseen by English Heritage.

Work on the east parapet of Framwelgate Bridge, dating from 1152 and rebuilt in 1400, proceeded unhindered and is now complete.

But steel cramps used in 1856 strengthening to the west parapet were found to have rusted, causing greater damage than anticipated.

Chris Tunstall, the county council's director of environment and technical services, said original estimates suggested 30 new stone blocks were required for the parapet due to damage from the steel cramps.

He said: "When the parapet was carefully taken down, we found more blocks were damaged as the steel cramps were literally welded into the stone.

"As a result, we will now need to order an extra 64 stones, which will take about five weeks to deliver."

Mr Tunstall said that, with agreement of English Heritage, a further 39 long blocks of original stone, damaged at both ends by the cramps, would be salvaged and used to replace smaller damaged blocks.

The measurements of each stone block were recorded at the start of the contract, because scheduled monument consent requires subsequent rebuilding to match the original structure.

Refurbishment of the two bridges, and similar work to a cantilever footbridge on the old Newton Cap bridge, near Bishop Auckland, County Durham, are part of a £400,000 package being carried out by contractor Lumsden and Carroll.