Pretoria - President Jacob Zuma benefitted "unduly" from a R246m state-funded security upgrade to his Nkandla homestead, the country's graft watchdog said on Wednesday in a damning report six weeks before the elections.
Public Protector Thuli Madonsela accused Zuma of conduct "inconsistent with his office" and said he should pay for some of the unnecessary renovations.
The findings are another blow to Zuma and may harm the ruling African National Congress (ANC) in the 7 May polls.
The Presidency said Zuma had been "consistently concerned about the allegations of impropriety" that have swirled around the upgrade.
He would study the report and give his response "in due course", the statement added.
Madonsela's 444-page summary of her two-year probe into the renovations at Nkandla painted a picture of systemic official incompetence and flouted tender procedures that Zuma never questioned.
"The President tacitly accepted the implementation of all measures at his residence and has unduly benefited from the enormous capital investment in the non-security installations at his private residence," Madonsela said.
She described the cost overruns as "exponential" and said ministers had handled the project in an "appalling manner".
When the Nkandla scandal first broke in late 2009, the cost was estimated at R65m. Despite intense public scrutiny since then, the bill ballooned to R246m as the project spiralled out of control.
The total amounts to eight times the money spent securing the home of former president Nelson Mandela and more than 1 000 times that spent on FW de Klerk.
Even though Madonsela's findings are in line with leaked excerpts to newspapers late last year, the ANC looks set to take a hit from voters angry at perceived corruption under Zuma and the country's shoddy public services.
The ANC, which has staunchly supported Zuma during previous corruption and personal imbroglios, cancelled a scheduled news conference, saying it needed more time to study Madonsela's findings. It will give its response at 10:00 on Thursday.
"This is negative for the ANC," said Nic Borain, an independent political analyst.
"They will lose votes as a result of this."
Underscoring the challenges facing the party, thousands of Numsa members sang anti-ANC songs as they marched through the streets of Johannesburg on Wednesday demanding greater workers' rights.