The crossing over the River Nidd at Cattal, between Harrogate and York, was sealed off on February 7 for eight weeks for repairs and maintenance.
But the original construction techniques have presented North Yorkshire County Council’s contractors with significant problems to overcome.
A council spokesman said: “This bridge is a listed building, and we are working closely with English Heritage to ensure that this vital repair work does not in any way alter the character of the structure.
“This has meant that many of the individual stones with which the bridge is being repaired have had to be dressed in a unique way.”
It is now anticipated that the bridge will reopen on April 21 following the repairs, which are expected to cost £78,000.
The spokesman added: “We apologise for any inconvenience caused by this slightly longer closure, but unfortunately it is inevitable.”
The bridge was built around 1800 on the site of the crossing of the River Nidd by the Roman road connecting Aldborough and Tadcaster.
Years of weathering, as well as numerous traffic accidents, had damaged the stonework which, if left untreated, would have posed a serious threat to the safety of the structure.