Legal action should be taken by the European Commission against Ireland if work recommences on the M50 at Carrickmines, Co Dublin, Friends of the Irish Environment (FIE) said today.
If action were taken it could lead to millions of euro in EU grants for the project being lost and heavy fines levied.
The FIE had asked the Commission to look into a number procedures they alleged had not been followed when permission was granted for the extension of the motorway though the historically significant site of Carrickmines Castle.
An RTE Prime Timeprogramme last night revealed leaked details of the Commission's expert report which indicated that regulations had been breached.
Earlier this year, a European Parliament Committee was told that funding of up to €75 million could be withheld if the Environmental Impact Study (EIS) for the area around the castle site was not in line EU requirements.
Campaigners claimed the study excluded key information and it emerged last month that the National Museum of Ireland would not have agreed with the issuing of an excavation licence for the Castle had it been better informed about the site.
It has also emerged that the National Monuments Service (NMS) had raised concerns in July 1997 about the inadequacy of the draft EIS.
Mr Tony Lowes of the FIE today said their position had been vindicated, saying "early and clear warnings by qualified professionals" were ignored.
"The overruns and controversy that has dogged this site are due entirely to decision makers ignoring professional advice," said Mr Lowes.
"The Commission's obligation extends beyond the withholding of funds. If work resumes, the Commission must bring proceedings against Ireland to ensure compliance with European law," he added.