Mr Proinsias De Rossa MEP has called for a full inquiry into the circumstances of the route selection of the South Eastern Motorway, in south Co Dublin.
Mr De Rossa also criticised the Minister for Transport, Mr Brennan, for making "the wrong decision" in allowing the motorway route through the ruins of Carrickmines Castle.
He also vowed that the European Parliament Petitions Committee, of which he is vice-chairman, would investigate the issue.
Commenting on Mr Brennan's expression that he was "not comfortable" about taking the decision to continue with the work on the final leg of the M50, Mr De Rossa said it was clear the decision had been made for financial reasons "at the expense of an irreplaceable part of our heritage".
Dublin County Council had been warned about the presence of castle remains by An Foras Forbartha, responsible for planning and construction research in 1983.
This advice had been incorporated into the 1993 Dublin County Council Development Plan which showed the motorway at a remove to the south from Carrickmines.
However, on assuming responsibility for the area, Dún Laoghaire-Rathdown County Council changed the route of the motorway, against An Foras Forbartha's advice, as part of its redrafting of the 1998 County Development Plan.
"It now transpires that in official circles at least, the extent and importance of this site was well known and was taken into account in previous planning of road alignments" said Mr De Rossa.
He said this information was not generally known."This witholding of essential information constitutes in my view a clear breach of the Environmental Impact Assessment Directive. The Minister is, therefore, obliged in law to order a complete cessation of all works forthwith and to order the preparation of a new plan which will go through the planning process."
Mr De Rossa said he had written to the EU Commission asking it to investigate "probable" breaches of European law. If found to be the case, EU funding for the road would likely be withdrawn, he said.
