Police in South Africa have opened an inquest into the sudden death of a politician who blew the whistle on alleged assassinations and corruption linked to the building of a World Cup stadium.

James Nkambule collapsed and died at his home in Mpumalanga province last week. A confidential autopsy report said the 37-year-old was poisoned, according to South Africa's Sunday Times .

Nkambule was reportedly the ninth local official in the past two years to be murdered or die in suspicious circumstances, allegedly for standing in the way of the tender process for the R1.3bn (£118m) Mbombela stadium in Nelspruit.

He claimed that corrupt politicians were behind a "hit list" in Mpumalanga and gave a copy to journalists and police. He said a Mozambican hit man known as "Josh" had confessed his involvement in the killings. Nkambule was attempting to get Josh into a witness protection programme, the Sunday Times said.

The autopsy, conducted by Mpumalanga's chief medical officer, Dr Gantcho Gantchev, concluded Nkambule's death was unnatural, the paper added. Gantchev described "white foamy material" in Nkambule's throat and windpipe, and about 30ml of brownish fluid suggestive of "poison ingestion" in his stomach. "They killed him, they killed my dad," Nkambule's daughter, Buhle Nkambule, was quoted as saying.

Police declined to comment on the claims of poisoning but confirmed today that an inquest docket had been opened. Captain Leonard Hlathi said: "We've taken some body tissue and we've sent it to forensics for further investigation and determination of the cause of death."

Nkambule was a controversial figure known to have enemies. At the time of his death Nkambule was facing charges of defeating the ends of justice, conspiracy to commit a crime, incitement and fraud. He claimed he was being used as a scapegoat to divert attention from "the person who is behind the killings in the province."

Speaking in June after an appearance at the Nelspruit magistrate's court, he insisted that he would not "shut up" until the truth of the assassinations was told.

Nkambule fell out with the governing African National Congress after accusing leading members of plotting to oust the then president Thabo Mbeki in 2002. He was branded a liar and later apologised. He was among rebels who joined the breakaway Congress of the People (Cope).

Cope has demanded a thorough investigation into Nkambule's death. "Cope calls on the police to investigate this death properly and to deploy all the necessary resources to ascertain whether his death was in fact a murder," spokesperson Philip Dexter said.

"For far too long, Mpumalanga has been the home and base to gangsters, thieves and alleged murderers masquerading as politicians. These must be exposed and dealt with harshly in terms of the law."

The Mbombela stadium was among the most controversial [http://www.theguardian.com/football/blog/2010/jun/03/nelspruit-world-cup] of South Africa's World Cup venues. It forced the relocation of two schools and caused angry protests in a neighbouring township. It hosted four matches for almost £30m a game or £327,500 a minute.