A law firm has gone to the US Supreme Court in "a last-ditch effort" to save a 40-foot-tall cross memorialising soldiers who died in World War I that sits at a busy intersection in the Washington suburb of Bladensburg.

Last October, the 4th US Circuit Court of Appeals based in Richmond, Virginia, ruled 2-1 that the 93-year-old monument is unconstitutional and must be removed or destroyed. "(It) has the primary effect of endorsing religion and excessively entangles the government in religion."

"The use of the cross reflected the custom in Europe during World War I," the Thomas More Law Center of Ann Arbor, Michigan, said. It filed its brief with the US Supreme Court on July 27.