Singapore Prime Minister Lee Hsien Loong threatened Sunday the editor of an online publication with libel for repeating "false" allegations related to a bitter feud in the city-state's founding family.

In a letter to Terry Xu, chief editor of The Online Citizen (TOC), Lee's press secretary demanded that the article on its website and Facebook page be taken down immediately.

The letter also asked TOC to issue a "full and unconditional apology" by Wednesday.

Should Xu fail to comply, "PM Lee will have no choice but to hand the matter over to his lawyers to sue to enforce his full rights in law," said the letter signed by his press secretary.

Singapore is admired worldwide for its economic prosperity, but its leaders are criticised for curbs on civil rights, including using libel suits to silence critics.

The TOC is known for publishing articles critical of the long-ruling People's Action Party in a country where the mainstream media is widely seen as pro-government.

Lee, 67, is embroiled in a public row with his younger brother Lee Hsien Yang and sister Lee Wei Ling over the fate of a family house following the death in 2015 of their father, Singapore's founding leader Lee Kuan Yew.

The Lee patriarch had wanted the century-old bungalow demolished after his death to avoid the creation of a personality cult.

But the prime minister's siblings accused their brother of blocking the house's demolition to capitalise on their father's legacy for his own political agenda.

Premier Lee has denied the allegations, but said he had chosen not to sue his siblings as doing so would further taint their parents' names.

The article in question was a TOC commentary last month on a Facebook post by Premier Lee's wife Ho Ching in which she shared a piece discussing why it was right sometimes to cut off ties with "toxic family members".

TOC referred readers to the family feud, which erupted in 2017 after the siblings aired their allegations on social media.

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The letter accused TOC of repeating the "false" and "libelous" allegations.

While Lee chose not to sue his siblings, this "should not be misinterpreted by others as free licence to repeat and spread false and defamatory allegations against him," it warned.

Xu could not immediately be reached for comment.

The article was no longer accessible on the TOC website but it could still be seen on its Facebook page.