"Whatever I do [since leaving the presidency] in no way involves me in the internal politics of the ANC or the functioning of the government of South Africa," Mbeki said.
"I appeal that nobody should abuse or cite my name falsely to promote their partisan cause, including how the 2009 ANC election campaign would be conducted."
Mbeki's letter was sent to Jacob Zuma, the ANC leader and a longtime Mbeki rival, earlier in the month but the publication of a leaked copy of the letter comes on the eve of a convention aimed at organising a breakaway faction within the ANC.
Zuma took control of the ANC from Mbeki during the party's conference last December, and his allies in the leadership last month pushed Mbeki to resign as president just months before the end of his second term after he was accused of meddling in a corruption case brought against Zuma.
The infighting between Zuma and Mbeki has exposed deep splits within the ANC, which had led the struggle to end apartheid in South Africa and has since dominated the country's politics.
Mosiuoa Lekota, a former ANC chairman, officially resigned from the party on Friday. The ANC had suspended him three weeks ago after he suggested that a new party be formed.
He and his allies say the convention this weekend will lay the groundwork for a new political party, which he says could challenge the ANC in elections next year, when Zuma is widely expected to become president.
The main opposition Democratic Alliance has said it will attend the convention.
In a full-page advert in The Star, convention organisers decried the "rapid degeneration of the political morality and value systems" of the ANC.
The advert also calls for donations for the new party, which faces a financial battle to put together an election campaign for polls just months away.