Environment Minister Mark H Durkan had no legal power to approve a major new planning blueprint for greater Belfast, a High Court judge ruled.
Mr Justice Treacy held that Mark H Durkan acted unilaterally and unlawfully in authorising the Belfast Metropolitan Area Plan (BMAP) without securing consent from Executive colleagues.
His verdict came in a challenge to the SDLP Environment Minister's decision brought by former DUP Enterprise Minister Arlene Foster.
The planning framework, adopted in September 2014, has implications for securing the first John Lewis store in Northern Ireland.
But the judge backed claims that because its significance stretched across departmental responsibilities it needed approval from the Stormont cabinet.
He said: "A Minister has no power to take a decision in violation of the Ministerial Code relating to the obligation to bring to the Executive committee any matter that requires to be considered by it for discussion and agreement by reason of being cross-cutting, significant or controversial.
"In the present case, the Minister, having failed to achieve any agreement at the Executive sub-group, acted unilaterally and unlawfully by authorising and directing the Department to adopt the BMAP without informing the Executive until after the event and despite objections having been raised by other Ministers."
A decision on what remedy to grant in the case will be taken at a later date.
BMAP identifies zones for retail, residential and commercial development across the city and outlying areas such as Carrickfergus, Lisburn, Newtownabbey and north Down.
Among the most contentious aspects of the blueprint is a retail zoning which restricts future expansion at the Sprucefield shopping centre to bulky goods only.
That would stop a long-proposed John Lewis store from being built there.
Mrs Foster, now the First Minister, issued proceedings amid claims her colleague in the power-sharing administration breached the ministerial code.
Mr Durkan insisted efforts were made to get the issue on the agenda at Executive meetings.
During the hearing it was confirmed that the legal action involved a disagreement split down party political lines.
The DUP is opposed to the restrictions adopted by the SDLP Minister in BMAP, the judge was told.
Counsel for Mrs Foster repeatedly argued that the planning framework was a cross-cutting, controversial matter which needed the agreement of the whole Executive.
He claimed "battle lines were well drawn" in the debate over allowing unrestricted retail development at Sprucefield, with the potential impact on town and city centre shopping.
Calling into question the extent to which Mr Durkan showed flexibility, he alleged the Environment Minister was only interested in securing approval for his own pre-determined outcome.
Mr Justice Treacy was told seven of the other 10 Stormont departments were concerned enough about BMAP to want to take part in a special executive sub-group set up to deal with the issue.
Mr Durkan's barrister contended that attempts were made to coerce him into ignoring his legal duties in dealing with the planning blueprint.
He also claimed the Environment Minister was put under pressure by DUP ministerial colleagues over a policy which effectively blocked attempts to build a John Lewis store.
Ultimately, however, the judge held that the decision did cut across responsibilities of others in the Executive under the terms of the 1998 Northern Ireland Act.
He confirmed: "It was therefore a function of the Executive committee to discuss and agree upon it, rather than for the respondent to act unilaterally."