JOHANNESBURG, Sept 17 (Reuters) - South Africa"s top court on Friday dismissed a bid by former president Jacob Zuma to overturn his 15-month jail sentence for contempt of court after he ignored instructions to participate in a corruption inquiry. The jail sentence came in June after Zuma failed to testify at an inquiry into graft during his nine-year rule, in a case widely viewed as test of post-apartheid South Africa"s ability to enforce rule of law, particularly against the powerful. Zuma, recuperating in hospital after surgery for an undisclosed illness, asked the court in July to revoke its sentence for contempt, arguing it was excessive and that jail would endanger his health and life. "The application for rescission is dismissed," Justice Sisi Khampepe said as she read the majority decision, which included an order for Zuma to pay costs. It was the latest legal setback for the 79-year-old anti-apartheid veteran from the ruling African National Congress, whose presidency between 2009-2018 was marred by widespread allegations of graft and malfeasance. He denies wrongdoing. "Obviously the foundation is disappointed with this judgement," Mzwanele Manyi, spokesman for the JG Zuma Foundation, said in response. NEW TRIAL STARTING Zuma"s jailing on July 7, after handing himself over to police at the last minute, triggered some of the worst riots and looting in decades, with more than 300 people killed and thousands of businesses pillaged and razed. The violence, which President Cyril Ramaphosa described as a "failed insurrection", was also fuelled by frustration among largely Black communities still in squalid conditions long after the ANC swept to power in South Africa"s first democratic elections in 1994. read more A former senior intelligence operative with the ANC"s then banned military wing uMkhonto we Sizwe before rising to the highest office, Zuma says he is the victim of a political witchhunt and that acting Chief Justice Raymond Zondo is biased. Zondo served as chairman of the graft inquiry. The department of correctional services placed Zuma on medical parole earlier this month after surgery following his hospitalisation in August. That decision is being challenged by the opposition Democratic Alliance. read more Ousted as president by Ramaphosa in 2018, Zuma faces a separate corruption trial linked to his sacking as deputy president in 2005, when he was implicated in a $2 billion government arms deal. That long-delayed trial against Zuma, who denies multiple charges including corruption, racketeering and money laundering, continues next week. read more