A South African court has ruled that President Cyril Ramaphosa's official crowning of the new Zulu king last year was "unlawful and invalid".
The court has ordered him to set up an inquiry into whether King Misuzulu ka Zwelithini's accession to the throne took place in line with customary laws.
It follows a legal challenge by the king's half-brother, Prince Simakade Zulu, who says he is the rightful heir.
Their father's death in 2021 sparked a bitter family feud over the succession.
On the throne for more than 50 years, the late King Goodwill Zwelithini was the longest-reigning Zulu monarch and had six wives and at least 28 children.
King Misuzulu appeared to have won the succession battle - especially after his traditional coronation in August 2022 was followed by a state ceremony two months later when Mr Ramaphosa gave him a certificate of recognition in front of tens of thousands of people.
Under South African law, the president gives official recognition to the new king, paving the way for him to be treated like a constitutional monarch and to be funded by the government.
The king also controls vast tracts of land under South African law.
Prince Simakade had argued that Mr Ramaphosa's recognition of his younger half-brother as the monarch was rushed and failed to follow proper traditional and legal procedures.
Mr Ramaphosa has not yet indicated whether he plans to accept the ruling or to challenge it in a higher court.
His spokesman said his legal team was still studying the ruling handed down by the Pretoria high court on Monday.
The court stressed that it was not determining who was the rightful king, but whether the president had followed proper procedures before recognising King Misuzulu as the monarch.
It said Mr Ramaphosa had failed to comply with the law, which required him to order an investigation into objections to the accession.
The office of the 49-year-old Zulu monarch has not yet commented on the ruling.
It has split public opinion in South Africa, and is likely to plunge the Zulu royal family into deeper turmoil.
Some fear that that it could inflame tensions in a monarchy that has been hit by claims of poisoning and killings since the death of the previous king.
King Misuzulu's supporters maintain that he is still the rightful king, including the leader of South Africa's third biggest political party, Julius Malema.
"King, my undisputed King. Bayede [Hail the King]," the Economic Freedom Fighters (EFF) leader said after the ruling.
However, some supporters of Prince Simakade are confident that he will eventually become king, seeing the ruling as the first step in helping him claim the throne.
Under Zulu custom, the eldest son does not automatically become king, and there have been vicious power struggles for the throne in the past as well.
South Africa has eight traditional monarchs officially recognised by the government, all funded by taxpayers.
BBC