JOHANNESBURG (Reuters) - The South African Communist Party said its investigators have found no credible evidence a missing R500,000 donation, apparently given to the SACP's general secretary, ever existed.

Earlier this year, a South African businessman filed a complaint against the SACP relating to a donation he said he gave to party member and labour federation COSATU President Willie Madisha.

Madisha says he received the money and delivered it to General-Secretary Blade Nzimande in 2002. But Nzimande has denied receiving the donation.

SACP national treasurer, Phumulo Masualle, said a task force charged with investigating the missing R500,000 could not find any credible evidence that such a donation ever existed.

SACP Chairman Gwede Mantashe said on Sunday the claim was part of an ongoing attack against the party by detractors within the ruling African National Congress (ANC) alliance.

"There is a sustained attack on the Communist party and that attack is ideological ... there is a polluted political environment in the alliance," Mantashe told reporters at the media briefing.

The ANC and alliance partners, SACP and the Congress of South African Trade Unions (COSATU), have been at odds over economic policy, which has fed into the ANC succession battle.

The SACP and the COSATU labour group accuse the ANC -- under the leadership of President Thabo Mbeki -- of failing to address widespread poverty and unemployment with its centrist policies.

COSATU and the SACP have backed former Deputy President Jacob Zuma, who was fired two years ago in a corruption scandal, as the next ANC president when the party chooses a new leader in December.

Traditionally the ANC president is also the state president. The party has garnered two thirds of the electorate since South Africa's first democratic election in 1994.