An appeals court in the US has upheld an order halting work on President Donald Trump's prized White House ballroom, setting up another battle in the Supreme Court over the limits of presidential authority.
The US Court of Appeals for the DC Circuit, which had allowed construction to continue while it considered the case, agreed on Friday with a lower court that such a massive White House overhaul required the explicit consent of Congress.
It stayed its own decision for two weeks to give Trump time to ask the Supreme Court to intervene.
The US president lashed out after the 2-1 ruling, calling it "horrendous, politically motivated, and unlawful."
The president vowed in a lengthy social media post to immediately appeal to the Supreme Court, while reiterating his argument that the entire project represented a matter of national security.
Trump initially said the construction of the ballroom was necessary to host galas, but has since pointed to the concurrent addition of security features.
Under the district judge's initial order, work was to be allowed to continue only on a limited set of structures, including those that were underground.
"This holding has nothing at all to do with whether the proposed ballroom is desirable, or not, as a matter of policy. This ruling does not even necessarily mean that the Defendants may not ultimately construct the ballroom," the majority wrote Friday.
"What it does mean is that the Defendants may not do so during the district court's expeditious litigation without securing Congress's authorisation, as the Constitution and laws require."
Trump's Washington makeover
Trump has repeatedly complained that the White House lacks sufficient space for grand galas such as state dinners.
Work on the ballroom project began last year with the demolition of the White House's entire historic East Wing.
Since then, photos have shown significant work on a basement complex, with some concrete walls rising up above ground.
Trump has insisted that the ballroom will be privately financed.
While the White House initially cited a $200 million price tag for the project, it quickly doubled the estimate to $400 million.
The ballroom is just one of Trump's projects to leave his mark on Washington by the time he finishes his term in January 2029, along with a renovation of the Lincoln Memorial Reflecting Pool, as well as the construction of a gigantic arch.
While some of the projects have faced legal challenges, the ballroom project would be the first to reach the Supreme Court.
