A letter from former president Thabo Mbeki to President Jacob Zuma was intended for the African National Congress’s (ANC's) leadership only, and not for the general public.
The Thabo Mbeki Foundation on Monday said it did not leak the letter to the media last week.
Mbeki wrote to Zuma, imploring him to have talks with the 101 ANC veterans who had publicly expressed concerns about his leadership.
The foundation said the letter was delivered to the party's secretary general, Gwede Mantashe.
"Regrettably, the letter was subsequently leaked to the media by persons we do not know, which then published it during the morning of November 3. This suggests that the print media would have been in the possession of the letter by November 2. The letter was also widely circulated on November 3 through the social media."
The foundation published the letter on its website following the leak.
"The letter was intended for President Zuma and the leadership of the ANC, not for public distribution. If it had been intended for such distribution as an 'open letter', there would have been no need to ensure its secure delivery directly to the ANC SG at Luthuli House."
The foundation said it believed the issues raised in the letter had wider national implications.
"President Zuma and the ANC leadership should focus on the contents of the letter with the requisite seriousness," the foundation said.