A Stormont minister has expressed disappointment after a High Court judge quashed his decision to give the green light to a £330 million dual carriageway project.
Roads Minister Danny Kennedy said he would now consider whether to appeal Mr Justice Stephens' ruling on the controversial plans for the A5.
The construction on two stretches of the proposed route between Londonderry and Aughnacloy in Co Tyrone formed part of a cross-border scheme to develop a new transport corridor between Dublin and the north west of Ireland.
The project has already been dogged by problems, with the Irish Government having reduced its initial financial commitment due the economic situation south of the border.
Last month the judge said he was minded to quash the minister's decision to proceed with the first two sections in Northern Ireland - from New Buildings to north of Strabane in Co Derry and from south of Omagh to Ballygawley in Co Tyrone - after finding in favour of a lobby group that had challenged it by means of judicial review.
Justice Stephens upheld one of what were a number of grounds of challenge submitted by the Alternative A5 Alliance - namely that Mr Kennedy's department had not carried out an appropriate Habitats Directive assessment on the potential impact on the River Foyle and River Finn special areas of conservation.
At Belfast High Court the judge confirmed he was quashing the minister's decision, but allowed seven days for an appeal to be lodged.
"Today's ruling is disappointing but my key priority now is to consider the merits of appealing and to find a resolution to this situation as quickly as possible," said Mr Kennedy afterwards
"I also intend to discuss this with Executive colleagues on Thursday."
Whatever the department's decision, the A5 project is certain to face a lengthy delay as a result of the court's ruling.