Nine members of the Kennedy family have joined litigation in Washington seeking to halt President Donald Trump's planned 250-foot triumphal arch, arguing that the monument would cast a shadow over the graves of their relatives at Arlington National Cemetery. The family moved on Tuesday to join the case before construction begins.

The Kennedys argue the arch would block the solemn sightline from the eternal flame at President John F. Kennedy's gravesite, across Memorial Bridge to the Lincoln Memorial, destroying an essential element of the gravesite's design. Rory and Maxwell Kennedy, children of Robert F. and Ethel Kennedy, said in a statement that an arch casting shadows across the graves of their father, mother and uncles, and of so many who served the country, would also cast a shadow over democratic principles and the safeguards put in place to protect the ground. The family argues that only Congress has the constitutional power to authorise an intrusion of this magnitude.

The arch, proposed to mark the 250th anniversary of the United States, has advanced in fits and starts since its unveiling last year. Its various proposed designs have featured a flame, gilded angels and a sniper's nest. Last week it received final approval from a commission allied with Trump, but within hours US District Judge Tanya Chutkan issued a 14-day restraining order, accompanied by a 33-page opinion revisiting the project's chief legal and practical problems, including whether it required congressional approval, the costs it would impose on Virginia and the District of Columbia, and disruption to traffic around Memorial Circle.

A White House spokesman, Davis Ingle, called it unfortunate that members of the Kennedy family had joined the litigation, saying the arch was intended to honour those resting in Arlington, and that the courts should not be used to politicise a structure designed to unite the nation. He said the administration would continue to make its case and was confident it would be vindicated.