Former President Thabo Mbeki is a "conniving" person who betrayed the legacy of struggle icon Nelson Mandela, the African National Congress's (ANC's) election head said in an open letter published on Tuesday.

"[Former President] Mandela handed you a vibrant and united ANC, yet at the twilight of your Presidency, you chose to betray everything that Mandela and those that came before him stood for, struggled for, and laid down their lives for," said Fikile Mbalula, campaign head of the ANC.

"In a moment of intoxication with power, you forgot Madiba's wise counsel and allowed our glorious movement to stumble on the edge of an abyss," said Mbalula, who was writing in his personal capacity.

"When your cabal was finally defeated in Polokwane because of its actions and underhanded tactics at securing a third term for you as a president of the ANC, they went into an elaborate conspiratorial mode, famously dubbed 'the fight-back strategy' which clearly carried your blessing."

Mbalula said that Mbeki decided to "spawn" the ANC breakaway party, the Congress of the People (Cope), after being defeated as ANC leader by Jacob Zuma at the Polokwane conference in December 2007.

"It is one's considered view that it was the failure of this strategy that led you and your lieutenants to spawn the so-called Congress of the People as a vehicle to fight the ANC and undermine its hegemony and legacy."

Mbalula, who is also a member of the ANC national working committee, said that the events of the past week had left him with "very little option but to address you directly on the matters at hand".

He referred to the release of transcripts of conversations between former prosecutions chief Bulelani Ngcuka and former Scorpions boss Leonard McCarthy, discussing the timing of re charging Zuma with fraud and corruption.

"How did the State apparatus become so embroiled in partisan politics that sought to rip our movement apart such that not even the highest office in the land had the political will to put brakes on the rot that was settling in?" asked Mbalula.

Acting National Prosecuting Authority (NPA) head Mokotedi Mpshe last week announced that the charges against Zuma will be dropped, saying that the transcripts were evidence of an "abuse of process".

Mpshe said that no concrete evidence could be found to support claims that Mbeki had manipulated the prosecuting process behind-the-scenes but many Zuma supporters believe that was the case.

"The NPA briefing finally brought closure to a painful episode of your reign both as President of the republic and president of the ANC, an episode one hopes will never come to pass ever again in the history of our movement," wrote Mbalula.

"It is a sad reality that the phenomenon we are dealing with today is a result of your actions of conniving, manipulating people and advancing politics of patronage.

"Despite the fact that you were a democratically elected President, you chose to run both the organisation and the country with a cabal which sought to commandeer everyone along your thinking and vision, which at times ran contrary to what the ANC stood for," said Mbalula.

Mbeki's spokesperson, Mukoni Ratshitanga, was not immediately available for comment.