The court ruled that inspectors examining the plan had given "no clear explanation" as to why, following a drop in the council's housing requirement figure, they had still decided there were "exceptional circumstances" justifying the amount of green belt to be released.
Leeds City Council's Site Allocations Plan (SAP) states that 66,000 new homes will need to be built in its area between 2012 and 2028.
In order to meet housing needs, 4,070 of those homes were allocated to green belt sites.
Aireborough Neighbourhood Development Forum yesterday succeeded in a judicial review challenge against the SAP.
Mrs Justice Lieven ruled that a "proper justification" had yet to be put forward for releasing swathes of the green belt for development.
According to the High Court judgement, the council is the second largest planning authority outside London, with a population of 784,000, and two thirds of its area is allocated as green belt.
The judge said the council had encountered recurrent difficulties in establishing that it has a five-year supply of deliverable housing land.
That, combined with problems in allocating land for development, had resulted in the council losing a number of planning appeals, the court noted.
The Leeds Core Strategy (LCS), adopted in 2014, identified a net housing requirement of 70,000 over the 16-year period, the court in London heard.
Although that was expected to be reduced to 66,000 by development of windfall sites, the court heard, it could not be met without developing areas of green belt.
However, the judge said the requirement has since had to be revised downwards in accordance with changes in national planning policy.
In a 2017 consultation document, entitled 'Planning for the right homes in the right places', the government set out the standard method for calculating housing need.
Following the standardised method approach, the housing requirement figure in respect of Leeds was 42,000 new homes over the 16-year period, very significantly less than the LCS's requirement for 66,000.
The judge said: "It became increasingly clear to the council that the figure of 66,000 was no longer sustainable and a materially smaller number of dwellings would now need to be delivered.
"The problem that the council faced was how to deal with this changing housing requirement whilst still trying to ensure that there was a development plan in place ensuring that it could be shown to have a five-year land supply."
Challenging the SAP - which was adopted by the council following an inquiry before two inspectors - the neighbourhood forum argued that it was based on the LCS's out-of-date assessment of housing requirements.
The council argued that, although the release of green belt sites might not be needed on the basis of "crude housing supply figures", it remained desirable as a means of ensuring a fair geographical spread and distribution of housing development.
Upholding the forum's complaint, however, the judge said: "It seems to me that there is a good deal of ex post facto justification in the argument being put (by the council) in order to overcome the problem with the figures and the lack of reasoning as to the green belt release."
The judge said that the inspectors' report made no mention of a "policy imperative" to spread development across the council's area, nor to a need to balance allocations between urban and rural land.
The judgement said that the forum and other objectors had "strongly" submitted to the inspectors "that green belt land should not be released because...there was no longer a need for green belt release and thus no longer exceptional circumstances."
That was probably the most important controversial issue for the inspectors to decide, but their reasoning on the issue was "very far from clear", the judge added.
"There is no clear explanation from the inspectors as to why, in the light of this drop in the requirement figure, they still decided there were exceptional circumstances justifying the level of green belt release in the SAP.
"The inspectors had to take the up to date position in respect of all material considerations and that must include the actual level of housing requirement if the policy had become out of date."
The inspectors' error of law in failing to give adequate reasons had prejudiced the forum "both by the simple fact that it is unclear what the inspectors did consider to be the exceptional circumstances, but also by the fact that the end result is the loss of a significant quantum of green belt land which, on one analysis, was not properly justified in terms of national policy."
Four of seven grounds raised in the claim were rejected or not allowed to proceed by the judge. The three grounds allowed related to legal errors on the part of the planning inspectors.
According to a statement issued by Leeds City Council, these errors identified by the judge related to "legally deficient reasons given in their report on: justifying the release of the specific green belt sites and on site selection process; and an error of fact relating to the calculated increase in supply of housing (mainly in the city centre) during the process".
Leeds City Council chief planning officer David Feeney said: "I am heartened that the judge has not found any legal errors with the council's approach to preparing the plan.
"The council has been clear from the start that the SAP is about meeting a fair distribution of housing needs (including affordable housing for local people) across the whole of the district, enabling new homes to be built in areas that meet local need and to avoid over-development in specific areas.
"The council hopes that the errors of law can be remedied, so as to avoid a return to speculative development and give continued certainty to investors in the city."
A spokesman for the council said that relief in the case has not yet been determined and the court has invited further submissions from all parties.
This article was changed at 15:30 on 09/06/20 to correct an error which stated that the plan in question had proposed to deallocate enough land for 12,481 homes. In fact, the plan under challenge proposed the deallocate enough land for 4,070 homes. The 12,481 figure was from an earlier draft iteration of the final plan.
Aireborough Neighbourhood Development Forum v Leeds City Council. Case Number: CO/3279/2019