Meath County Council and the National Roads Authority have been issued with a solicitor's letter threatening legal action if they proceed with excavations in the Tara-Skryne valley for the proposed M3 motorway.
The letter, issued on behalf of Mr Vincent Salafia, spokesman for a campaign to save the archaeologically rich valley, said test excavations were scheduled to start today. It seeks a meeting to discuss the matter.
"The monuments in this valley are an integral part of the archaeological complex that forms the Hill of Tara national monument," the letter said.
It claimed the proposed works would be illegal under the National Monuments Acts, and added that Mr Salafia was ready to institute proceedings "immediately" seeking an injunction to prohibit any works in the valley.
Academic experts on Tara have described the valley as a unique landscape that should be subject to the highest level of protection.
Dr Conor Newman and Mr Joe Fenwick, of NUI Galway, and Ms Edel Bhreatnach of UCD said that clearing archaeological sites from the path of a motorway did not constitute a research plan.
They called for Tara to be designated a World Heritage Site.
Dr Muireann Ni Bhrolcháin, spokeswoman for the campaign group Save the Tara-Skryne Valley, said it had been "proven that there are huge scientific problems with the current plans for the M3. We want to begin formal negotiations, not litigation", she added.