Mbeki's campaign for more G8 aid to Africa is hit by struggle over his scandal-ridden deputy

President Thabo Mbeki of South Africa, who is trying to convince G8 leaders that more aid to Africa will not be pilfered by the continent's corrupt rulers, is facing the toughest decision of his presidency: what to do with his own embattled deputy, Jacob Zuma.

Mr Zuma has refused to resign after a South African court last week imposed a 15-year jail sentence on a businessman accused of paying him more than £100,000 in exchange for business favours. Not only has Mr Mbeki so far refused to fire his deputy, straight after the verdict he named Mr Zuma as his stand-in while he went to Chile.

The South African leader has diligently lobbied G8 leaders to back Tony Blair's project for rich countries to fork out another $25bn in annual aid to Africa. The Prime Minister's initiative rests on the assumption that Africa has developed the capacity to absorb huge amounts of aid without it being lost to the continent's legendary corruption.

As leader of Africa's largest economy and most stable state, many say, Mr Mbeki must show that corruption will not be tolerated, helping to still the many critics of Mr Blair's plan in G8 countries. But firing Mr Zuma risks a split in the ruling ANC.

"Few people would choose to be Deputy President Zuma right now. But fewer still would want to be in the position of President Mbeki," wrote one political analyst, Vukani Mde.

Unlike the intellectual Mr Mbeki, who is considered aloof and cold by the party rank and file, the charming Mr Zuma commands grassroots support in the ANC. Although he has no formal education, he is a skilled negotiator, brokering a deal that ended widespread violence in KwaZulu Natal and saved the country's first free elections in 1994.

In line with ANC tradition, Mr Zuma was seen as the automatic successor to Mr Mbeki when the President's term ends in 2008, and his supporters insist they will fight any attempt to block his succession. The South African Communist Party and the Congress of South African Trade Unions, Mr Mbeki's partners in his ruling tripartite alliance, have declared unconditional support for his deputy.

The influential ANC youth league has gone even further, attacking the integrity of the judge in the corruption case. Hillary Squires, a former justice minister under the Ian Smith regime in UDI-era Rhodesia, was described as an "old, white apartheid judge" recruited in a "shady scheme" to block Mr Zuma's ascent to the presidency.

Even Nelson Mandela has come out in support of the Deputy President, after Mr Zuma paid what was described as a "courtesy call" on the country's most revered figure.

Dismayed speculation in the press - Business Day said it was "one of the worst calls of his long political life" - had it that Mr Mandela was seeking to preserve ANC unity above all. But there is no doubt that Mr Zuma has succeeded in a charm offensive to reinforce his support in the party and complicate any decision to fire him.

Even commentators who oppose Mr Mbeki strongly have acknowledged his dilemma. "If Mbeki were to act as swiftly as many are demanding, he could cause an almighty rumpus in the ANC," wrote the usually anti-Mbeki Citizen newspaper in an editorial. "Therefore we expect a slow, methodical exit for Zuma. But he must go or Mbeki's legacy as Africa's leading light will be ruined."

Instead of firing Mr Zuma tomorrow, as some reports are speculating, sources say Mr Mbeki might seek first to mobilise support within the party. Whether he can do that before the G8 summit at Gleneagles next month is another matter.