A High Court judge has confirmed he is quashing the decision to go ahead with a new £330m dual carriageway project
However, Mr Justice Stephens put his order on hold for one week.
He granted the Department for Regional Development a seven-day stay to lodge any appeal to his ruling on the A5.
He also awarded legal costs to the group of farmers, landowners and supporters seeking to block work on 85km stretches between Derry and Aughnacloy, County Tyrone.
Last month, the Alternative A5 Alliance won their judicial review challenge to the planned new route due to a breach of a habitats directive.
The scheme, the largest of its kind ever in Northern Ireland, forms part of a proposed key cross-border business route linking Dublin and the north west.
Following the verdict, lawyers for the department sought to have the court order put on hold.
They wanted time to meet a requirement to carry out an appropriate assessment required under the directive.
But Mr Justice Stephens refused, citing the potential for a public inquiry and potential scope for legal confusion.
"The appropriate course in the exercise of my discretion is for the orders to be quashed," he confirmed.
However, acknowledging the potential for an appeal of his decision in the case, the judge agreed to a more limited delay.
"The applicants are entitled to plan their own businesses and their own lives," he said.
"They should not be left in any doubt about what may or may not occur in circumstances such as these where the department has acted unlawfully in breach of a habitats directive.
"I'm prepared to grant a stay until 12 noon on April 15 2013."
The legal challenge centred on the decision to press ahead with two sections of the route, for which the Stormont Executive has approved £280m.
Counsel for the Alliance, instructed by C&J Black Solicitors, claimed it has now become a different project, and that no proper environmental impact assessment (EIA) has been carried out.
In his first judgment last month, Mr Justice Stephens dismissed all but one of the campaign group's claims.
He held there had been a failure to carry out an appropriate assessment of the Rivers Foyle and Finn Special Areas of Conservation under the habitats directive.
'More harm than good'
Following the final determination a member of the Alternative A5 Alliance's committee expressed delight at the outcome.
Laura Kwasniewska said: "The decision has been quashed, albeit with a seven-day stay.
"We feel we have been vindicated and we are also relieved to have our costs paid."
She added: "This is a scheme that would have done much more harm than good, it's not in the public interest.
"It was never justified by either the traffic figures or indeed by the empty promises of economic growth, it would have been both costly and environmentally destructive."
She claimed ways of upgrading the existing A5 should now be explored.
"That would be far less costly and would deliver improved journey times and benefits for all the community."