South Africa’s Human Rights Commission is investigating reports that Zulu King Goodwill Zwelithini called gay people “rotten” during a speech.
The royal household has denied that the king made any homophobic comments – and has blamed “reckless translation”.
South Africa’s Times newspaper, which first carried the story, told the BBC it stands by its translation.
South Africa’s constitution specifically forbids discrimination on the grounds of sexual orientation – but homophobia is widespread and gay people complain they are often attacked.
The BBC’s Pumza Fihlani in Johannesburg says that as the leader of South Africa’s largest ethnic group, King Zwelithini is the most influential of the country’s many traditional rulers.
HRC spokesperson Vincent Moaga says the traditional king is respected by millions of South Africans – and they need “an accurate reflection of what he said”.
The HRC says it will be writing to the king and will demand an immediate retraction if he admits to making homophobic remarks.
“If it is indeed accurate that His Majesty, the Zulu king, made the utterances as reported, they constitute hate speech… and are inflammatory,” Mr Moaga said.
King Goodwill Zwelithini allegedly made the anti-gay remarks in rural eastern South Africa during a ceremony at the weekend to mark the Battle of Isandlwana – a famous 19th Century Zulu victory over British troops.
“Traditionally, there were no people who engaged in same-sex relationships,” The Times quoted the king as saying.
“There was nothing like that and if you do it, you must know that you are rotten,” King Goodwill said, according to the newspaper, adding: “I don’t care how you feel about it … same sex is not acceptable.”
But the king’s office says the newspaper reports were badly translated and the king’s meaning misconstrued.
“At no stage did His Majesty condemn gay relations or same relations,” spokesperson Prince Mbonisi Zulu told the Sapa news agency.
The king was referring to cases of male rape as a sign of moral decay, the spokesman said.
President Jacob Zuma – the first Zulu leader of modern South Africa – was also at the weekend ceremony and used the occasion to call on South Africans to end discrimination against gay people.
Last year’s brutal murder of 24-year-old gay activist Noxolo Nogwaza highlighted South Africa’s growing homophobia, correspondents say.