Northern Ireland's Environment Minister acted unlawfully in approving the redevelopment of Casement Park, a High Court judge in Belfast has ruled.
Mr Justice Horner held that the decision-making process around Mark H Durkan's decision to grant planning permission for the £77m Casement Park redevelopment was "irretrievably flawed".
The judge identified failures in the environmental impact assessment of increased facilities and an unrealistic reliance by the Department of the Environment (DoE) on an existing 32,600 capacity as a baseline for the project.
With fewer spectators normally attending matches at the current ground, the judge found that the effect of bigger crowds on the surrounding roads network was not properly examined.
He said: "This was both an inadequate and unlawful approach and it meant that the likely significant effects of full capacity attendances at the ground were not and could not be adequately assessed."
The judge also pointed to the minister never being told of PSNI concerns about safety issues around having 38,000 fans attending an event.
However, he did not quash the decision to give the green light to the redevelopment.
Instead, lawyers for the DoE, the GAA and a residents group who mounted the legal challenge are to make further submissions on the appropriate remedies.
Mr Justice Horner's verdict followed one of the longest judicial review applications to come before the courts in recent years.
As well as hearing 13 days of legal arguments, the judge visited the site in person to help in his assessment of the project's potential impact on the surrounding neighbourhood.
The proposed new stadium is to include fully modern facilities and corporate resources.
Funding for the development is being split, with £62m coming from the Department of Culture, Arts and Leisure and £15m from the GAA.
During the case lawyers for the residents described the scheme as an injustice which would be unacceptable anywhere else in Northern Ireland.
It would be impossible to construct a stadium on that scale which will visually integrate with the existing neighbourhood, it was claimed.
Central to their case was an allegation that the environmental assessment was defective because it relied on Casement Park's existing 32,600 capacity as a baseline for examining the environmental impact of an increase of 5,400 seats.
Mr Justice Horner was told the new stadium would be replacing a ground which hasn't come close to selling out a lower capacity in decades.
Police claims that emergency evacuation of a full house at redeveloped facilities could take up to 47 minutes were also explored.
But counsel for Northern Ireland’s Planning Service warned during the hearing that GAA fans will be stuck with a crumbling and decaying venue if the decision to approve a new arena is quashed.
The £62m of public money allocated to the redevelopment would then be returned to the Stormont Executive for spending elsewhere, the court heard.
Lawyers for the GAA insisted the sporting organisation would not abandon Casement Park if the planning permission is overturned.
However, the project's spiralling bill was also outlined.
The delay in starting full construction on the new Casement Park GAA stadium is said to be costing £60,000 a week
The overall bill for the redevelopment in west Belfast has now risen by £2.7m.
Counsel for the GAA also revealed that nearly £5m of preliminary work will be lost if the scheme is halted.
Since the hearing a redeveloped Casement Park has emerged as a potential venue in Ireland's bid to host the 2023 Rugby World Cup.
But as he began his judgement, Mr Justice Horner stressed: "The court refused to be influenced by financial and sporting advantages or disadvantages that may accrue to either party."
Members of MORA and Stormont Culture Minister Carál Ní Chuilín were among those packed into Queen's Bench Courtroom Number 2 for the verdict.
Setting out the grounds on which the challenge succeeded, the judge said the 32,600 spectator baseline or 'fall-back position' did not reflect the reality of normal match-day attendances.
He pointed out that the GAA never obtained planning rights for the present stadium to facilitate such a large number.
Safety issues with the present terracing meant it would be impossible to admit 32,000 people, the judge said.
The Roads Service did not carry out a traffic impact assessment of a capacity 38,000 crowd coming into the area, the court heard.
Having held that the department wrongly adopted the 32,600 baseline, Mr Justice Horner also described this approach as fundamentally flawed.
No determination was reached on whether the failure to inform the minister of police safety concerns was deliberate or accidental.
But the judge concluded that it denied Mr Durkan the opportunity to consider all relevant evidence.
On that basis, he ruled, his decision to grant planning permission was "irretrievably flawed".
Defects were also identified in the environmental survey, with no assessment of the impact on local residents of extra stadium facilities such as conference suites, bars, restaurants and car parking.
With a further failure to engage with home owners about mitigating the risk of any asbestos escaping from the site during demolition and building work, Mr Justice Horner confirmed the department had not carried out an assessment in accordance with the regulations.
A further hearing is now expected to decide the final outcome of the case.