The US Supreme Court has ruled that construction of a $400m (£297m) ballroom at the White House can go ahead, in a victory for President Donald Trump.
In a 5-4 ruling, the justices granted the Trump administration's emergency request, blocking lower court rulings that would have halted construction of an above-ground portion of the project.
But Chief Justice John Roberts, a conservative, sided with the three liberal justices to dissent, writing that "construction is likely unlawful" because Congress has not approved the project.
The National Trust for Historic Preservation filed the legal challenge, arguing that the president did not have unilateral authority to proceed with construction.
Trump demolished the historic East Wing of the executive residence last autumn to begin building the ballroom. The White House has said the project will be "substantially" finished by November and fully completed by August 2028.
On Friday, the high court had temporarily allowed construction of the 90,000 sq ft (8,400 sq m) ballroom to move forward while the justices considered the lawsuit aiming to halt it entirely.
In Monday's unsigned order, the five justices who cleared the way for construction said lower courts had "likely exceeded" their authority in blocking the ballroom.
While the majority did not decide on the legality of the East Wing project, they said the preservation group "lacks standing" to sue the government.
They ruled that the Trump administration had established it was likely to succeed in arguing that it would "suffer irreparable harm without a stay".
Chief Justice Roberts wrote in dissent: "Today's decision is no victory for the separation of powers."
Reacting to the ruling, Trump posted that he was "pleased" the "baseless" lawsuit had failed to halt construction.
"We are living in the Golden Age of America, and this Building will be one of the Greatest ever constructed in Washington, DC," he said.
Trump has long argued that the White House needs a larger events space, and more recently, that it was important for his security.
The Republican president emphasised the need for the ballroom after an alleged assassination attempt at the White House Correspondents' Association dinner in April.
But the National Trust for Historic Preservation argued that construction was causing irreversible damage to a historical landmark.
Its position stemmed from a declaration made by preservationist Alison Hoagland, who lives in Washington DC and serves on the trust's board of trustees.
Hoagland, who says she visits the neighbourhood once a month, asserted that she would "suffer both professional and personal injuries, including to my aesthetic, cultural and historical interests, if a ballroom of the proposed form and scale were constructed".
In the dissent, Roberts said the injuries claimed by Hoagland were sufficient to allow the lawsuit to go ahead.
The National Trust for Historic Preservation said it was disappointed in the court's decision, but welcomed the "strong dissent" by Roberts.
"Each President is a temporary steward of the People's House and does not have the unilateral authority to demolish and redesign it without the approval of Congress," it said in a statement.
The ballroom project, along with the helipad being constructed on the South Lawn, are just part of the president's wider effort to transform Washington's cityscape.
His plans include the renovation of the Lincoln Memorial Reflecting Pool and a planned "Arc de Trump" larger than the Arc de Triomphe in Paris.
