A furious South African President Jacob Zuma has called an urgent family meeting at his family homestead in Nkandla, in rural KwaZulu-Natal, this weekend to deal with a letter that alleges infidelity by his second wife, Nompumelelo Ntuli.

On Thursday, the Durban newspaper Ilanga reported that it had received a letter from "concerned family members" in which MaNtuli is accused of cheating on the president with her bodyguard, Phinda Thomo, who reportedly committed suicide earlier this year.

Recent media reports have suggested that MaNtuli, who has two children, is pregnant again - and the paternity of the unborn child is being questioned by the "concerned family members".

The letter reads: "... what is the mystery now and talked about by the family is whose child is it that she is carrying. Is it ubaba's (Zuma's) child or the ex-private bodyguard?"

The letter says that MaNtuli became pregnant when she and Zuma "were apart and not talking to each other".

Zuma, 68, returned last night from an official trip to India with MaNtuli, 35. The Presidency released a statement on Friday saying Zuma would not dignify "gossip" with a response.

The statement reads: "The Presidency has noted with great concern reports that violate the privacy of President Jacob Zuma and his wife, Mrs Nompumelelo Zuma. The reports appear to be part of an ongoing and malicious campaign to undermine the right of the president and his family to privacy and dignity."

The president's businessman nephew, Khulubuse Zuma, denied that the letter was written by members of the family.

"It is particularly shameful and alarming that the president's political enemies have now taken to masquerading as members of his family to spread malicious innuendo about him and his family," he said.

He said that the family "is united" in distancing itself from the letter.

But, according to insiders, an angry Zuma wants answers from members of his family and has convened an urgent meeting for late on Sunday or on Monday, at which the allegations will be discussed.

"The president is deeply concerned about these allegations," said an insider. "That is why the intelligence (services) and the police are now on this matter.

"He wants to know about the letter and how it ended up in the press."

The latest controversy is further evidence that not all is well in the president's polygamous family.

The first signs of a fissure emerged soon after Zuma's election as president, when MaNtuli and Zuma's third wife, Thobeka Madiba, were caught on camera shoving each other in what appeared to be a fight over who should stand next to him.

When Zuma married KaMadiba, in January, MaNtuli was conspicuous by her absence.

The letter alleges that she has had run-ins with the president's first wife, Sizakele Khumalo, and that their conflict resulted in MaNtuli moving out of the homestead in Nkandla and settling in Durban.

"We, as family, truly believe that ubaba made the biggest mistake of his life by taking MaNtuli as his second wife," the authors of the letter write. "As the family, we are all shocked because we don't know her ... We do feel embarrassed to be associated with somebody like MaNtuli and feel sorry for ubaba, who has to live with her for the rest of his life."

It was reported that MaNtuli paid a fine of a goat in April this year as a punishment for her alleged cheating.

Meanwhile, the Sunday Times has established that a top South African actor's previous relationship with MaNtuli has come back to haunt her.

Although the two were an item before she became involved with Zuma, the married actor has confirmed that they have a child together - although this is something he has yet to tell his wife.

Earlier this year, Zuma was embroiled in controversy when the Sunday Times revealed that he had fathered an illegitimate child with a daughter of South African soccer boss Irvin Khoza.