An Executive decision to proceed with the A5 road upgrade has been quashed at the High Court in Belfast.
Mr Justice McAlinden told the court that he was aware his ruling would bring “fresh anguish” to the families of those who lost loved ones on the road.
But he said the proposed scheme breached elements of the Climate Change Act 2022.
Infrastructure Minister Liz Kimmins was at the Royal Courts of Justice to hear the ruling on Monday.
The Northern Ireland Executive gave the green light for the long-awaited upgrade to the A5 in October.
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However, judicial review proceedings were brought against the Department for Infrastructure by nine applicants, including residents, farmers and landowners opposed to the £1.2 billion scheme.
There have been more than 50 deaths on the A5, which links Derry with Aughnacloy in Co Tyrone, since 2006.
A scheme to turn the road into a dual carriageway was first approved by the Executive in 2007 but it has been held up by legal challenges and uncertainty over funding.
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