Opposition politicians in South Africa are calling for a “thorough and transparent” investigation into a car crash that killed a controversial South African businessman accused of bribing dozens of top government officials.

Gavin Watson, 71, died when his Toyota Corolla hit a concrete bridge support in Johannesburg in the early hours of Monday morning as he was travelling alone to the city’s international airport.

Watson’s company has been at the centre of a judicial investigation into allegations of systematic graft that implicate scores of leading figures in the African National Congress (ANC) party, which has been in power since the end of the apartheid regime 25 years ago.

South Africa has faced a series of corruption scandals in recent years. Jacob Zuma was forced to step down as president in February last year after widespread allegations of wrongdoing, which he denies.

Revelations of graft have badly damaged the country’s international standing, undermined investor sentiment and destabilised politics. People within the deeply divided ANC have sought to use multiple inquiries to discredit opponents and derail reform.

The judicial investigation has heard testimony from whistleblowers alleging Watson’s logistics company, Bosasa, paid millions of dollars to officials in exchange for lucrative government contracts.

Former executives have said about $5m (£4m) was paid in bribes and some state officials received monthly payments. Others were given cash in Louis Vuitton handbags, Cartier pens, cars and houses, and free security equipment, and had school fees paid for their children. One was sent luxury food deliveries over Christmas.

A video surfaced in January that apparently showed Watson counting out huge sums in cash to be handed over as bribes.

Cyril Ramaphosa, the president, who was elected on the promise that he would sweep away corruption, was forced to admit unwittingly accepting a donation from Watson in his campaign to unseat his predecessor. Bosasa also paid Ramaphosa’s son Andile a consulting fee for “advisory services”.

Watson was due to testify at a separate inquiry into his tax affairs this week.

Police have opened a case for culpable homicide, an offence similar to manslaughter in British law. No reasons have been given for the decision.

Solly Malatsi, a spokesman for the Democratic Alliance, the biggest opposition party, said Watson’s death would hinder the graft investigation.

“It is vital … that even a whiff of foul play is dispelled. This is to ensure that other witnesses are not intimated by this incident and for the commission of inquiry to conduct its work without fear or favour,” he said.

Leaders of the radical leftwing Economic Freedom Fighters suggested Watson had been killed to stop him giving evidence to South Africa’s anti-corruption ombudsman, Busisiwe Mkhwebane.

Watson’s decision to leave his BMW SUV parked in the basement of the Bosasa offices and use a more modest vehicle has also fuelled suspicions of foul play.

One whistleblower said it was “very plausible” that Watson may have been trying to flee the country.

“If you consider the charges that would come against him, I mean, it would be very plausible,” said Angelo Agrizzi, a former Bosasa executive.

Watson comes from a family known for its role in the anti-apartheid movement and is the oldest of four brothers who broke apartheid laws to play rugby with black people. They also joined the banned ANC, facing repeated arrest and detention.

The family owned a highly successful chain of menswear shops and became power brokers in both politics and business as the apartheid regime crumbled in the early 1990s.

Bosasa garnered contracts worth $140m from the state between 2000 and 2016, providing services ranging from prison catering to fleet management and information technology.

James-Brent Styan, the co-author of the book The Bosasa Billions, told a local news website that Watson’s death would have an “immense” impact.

“It is known that Watson didn’t keep notes or records. He didn’t have a computer or an office … He was a central figure to uncovering the truth about all the allegations relating to Bosasa, and now he’s not here any more to tell his version of events,” Styan said.