The letter, dated 17 March and signed 'It's me', reveals how Mrs Mandela's relationship with Dali Mpofu - who at 30 is about half her age - helped wreck her marriage to Nelson Mandela. It discloses that at one point Mr Mandela, whom she calls by his family nickname 'Tata', refused to speak to her for five months because of the affair. The letter was authenticated using a copy of her handwriting.
Mrs Mandela lays bare her obsession with Mr Mpofu, who was part of her defence team during her trial for the kidnap and assault of a child activist and who later worked in the ANC's social-welfare department, which she headed.
Mrs Mandela refers to the mother of his child as a 'white hag' and accuses him of betraying her (Mrs Mandela) by 'running around fucking at the slightest emotional excuse'.
'The only time you have time to talk to me is about women ofeba nabo (a Xhosa term meaning 'women you are having sex with'), as you are doing right now. You are supposed to care so much for me that the fact that I haven't been speaking to Tata for five months now over you is no longer your concern.
'I keep telling you the situation is deteriorating at home, you are not bothered because you are satisfying yourself every night with a woman,' the letter said, adding: 'I won't be your bloody fool, Dali'.
Less than a month later, Mr Mandela announced his separation from his wife.
But it is likely to be a later section of the letter that Mrs Mandela will have most cause to regret. She admits giving 160,000 rand (about pounds 30,000) to Mr Mpofu, and talks of her fear of an investigation into the accounts of the social welfare department of the ANC.
'You think you can just wish away certain things Dali, not with me. I tell you I'm in trouble with the Simmonds Street a/c which reflects over R160,000 drawn over a period for you. You don't even bother to check how we can overcome this. I tell you (ANC lawyer Ismael) Ayob has been sent by Tata to get an accountant to investigate my a/c. I tell you Ntombi (a woman in the ANC welfare department) is gossiping about the cheques we used to ask her to cash in the name of the Dept and how I gave you all that money,' the letter says.
Two months later the ANC said it was investigating alleged fraud involving 400,000 rand while Mrs Mandela and Mr Mpofu were running the department. In May, Mr Mpofu was sacked by the ANC after Mrs Mandela resigned from the welfare department. He is suing for wrongful dismissal.
The newspapers which published the letter - the Sunday Star and the Sunday Times - said both people declined opportunities to comment.
Yesterday, telephone calls to Mrs Mandela's Soweto home were not answered, and Mr Mpofu's home number was not listed.
The ANC's information chief, Pallo Jordan, declined to comment on the letter other than to suggest it had been intercepted and leaked by government intelligence officials. However, there is no shortage of candidates within the ANC if the organisation wants to consider Mrs Mandela's other enemies.
The Zulu leader, Mangosuthu Buthelezi, called on Inkatha's youth wing to 'stop the ANC in its tracks' after the murder of 10 Inkatha supporters on Friday. Chief Buthelezi said the ANC was obsessed with seizing power. He subsequently denied that he was advocating violence.