Police say Ram Singh, 33, hanged himself at about 5am local time on Monday in Delhi’s high-security Tihar jail, but his family and defence lawyers believe he was murdered. An inquiry is under way.

Singh and five others were on trial at a fast-track court in Delhi and faced the death sentence. A sixth suspect is being tried by a juvenile court. They all deny the charges.

The attack took place on the bus Singh drove for a living. The victim and a 28-year-old male friend boarded the vehicle on a Sunday evening after seeing a film. They believed the bus was one of the many unlicensed carriers that fill the gaps left by Delhi’s public transport system.

Instead, they were attacked with an iron rod and the woman was repeatedly raped. After nearly an hour, they were pushed from the moving bus onto the road near the city’s airport. The woman died of massive internal injuries.

The attack caused global outrage, protests across India and calls for deep legal and policing reforms. It also led to a heated debate about the treatment of women.

Singh’s lawyer, VK Anand, told the BBC the police informed him of his client’s death.

"This is shocking news”, Anand said. “There were no circumstances to suggest that he would commit suicide. He was happy with the trial; it was going on smoothly”.

The murdered woman’s 20-year-old brother told Reuters “I'm not very thrilled with the news that he killed himself because I wanted him to be hanged ... publicly.

"Him dying on his own terms seems unfair. But, oh well, one is down. Hopefully the rest will wait for their death sentence”.

Singh’s father, Mangelal Sings, said his son had a badly injured hand and could not have hanged himself.

He also said his son had been raped in prison by other inmates and repeatedly threatened by prisoners and guards.

The case was due to resume today after recent delays. Reporting restrictions mean few details of the trial have been made public.

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