A divided federal appeals court ruled Friday that President Donald Trump is unlawfully constructing a new ballroom at the White House, setting up a fight that is now expected to land before the Supreme Court. The U.S. Court of Appeals for the District of Columbia Circuit found in a 2 to 1 decision that the roughly 90,000 square foot, $400 million project requires congressional approval that the administration never obtained.

The ruling affirms a modified preliminary injunction issued earlier by U.S. District Judge Richard Leon. Under the order, work on the ballroom must stop except for below ground construction, including installations the administration says are needed for security reasons. The decision is the latest setback in a legal fight that dates back to last year, when the administration demolished the White House’s East Wing to make room for the new structure.

The National Trust for Historic Preservation brought the underlying lawsuit in December, arguing the demolition and planned construction bypassed required historic preservation review. Friday’s ruling sided with that argument, finding the White House lacked unilateral authority to move forward with a project of this scale without lawmakers signing off.

President Trump said he would appeal the ruling directly to the Supreme Court, teeing up a high stakes showdown over presidential authority and how far a White House can go in altering federal property without congressional input. Legal analysts say the case could take months to resolve given the scope of the questions involved.

For now, construction crews are limited to underground work at the site where the East Wing once stood, while both sides prepare for the next stage of a legal battle that has already outlasted the original demolition by nearly a year. The White House has not said when it expects to file its appeal with the Supreme Court. Preservation advocates say the case could set a lasting precedent for how much authority any future administration has to alter historic federal buildings without lawmakers weighing in.