A suspected Viking settlement has been discovered along the planned route of the €300m Waterford City By-Pass.
The National Roads Authority has confirmed to RTÉ News that it is treating the site as one of 'special interest' and it could demand 'a significant amount' of additional expenditure.
The NRA says this site was located at Woodtown last August, and, following preliminary excavations, several artefacts were located which suggest it was a possible Viking settlement.
It is believed the planned road would affect one third of the site.
An NRA spokesman told RTÉ News that the NRA had been adopting a responsible approach by consulting with the Department of the Environment, the National Museum and the Heritage Council.
He added there had been no prior evidence of such a site, despite an in-depth planning process.
He said the NRA believes that the by-pass can and should go ahead because the routing cannot be changed without restarting the planning process from the beginning.
It argues the site should either be excavated and then built on, or 'preserved' - in other words, built over without excavation in order to protect what is underneath.