The mammoth Newhall Ranch development, a proposal to build more than 20,000 residences and 5 million square feet of office space over 2,500 acres in the Santa Clarita Valley, suffered a major setback in a court decision released Monday.

The California Supreme Court threw out an environmental impact report prepared for the project by the Army Corps of Engineers and the state’s Department of Fish and Wildlife. In its ruling, the court said the report was inadequate in its assessment of the project’s greenhouse gas emissions and its mitigation measures for a protected fish species.

Developer Newhall Land & Farming Co. of Valencia said it is reviewing the court’s decision and will work with the fish and wildlife department on the next steps in the legal process. “We remain committed to realizing the vision of Newhall Ranch and the significant benefits it promises for the economy and future of Los Angeles County,” the company said in a statement.

Environmental and Native American groups have filed multiple lawsuits at the state and federal level challenging the Newhall Ranch development since it won Los Angeles County approval more than a decade ago.

But the project appeared poised to finally move forward in March of last year, when a state court of appeals upheld the EIR. In July 2014, however, the state high court agreed to hear an appeal of that ruling.

In its Monday opinion, a five-member court majority ruled that the fish and wildlife department “abused its discretion by making the determination, without the support of substantial evidence, that the project’s greenhouse gas emissions would have no significant impact, and in imposing biological resource mitigation measures that call for the trapping and transplantation of a fully protected fish species.” The justices reversed the appeals court’s conclusion and sent the case back for reexamination at the lower court level, a process which is likely to delay the project for several years.