The main suspect in the gang rape and fatal beating of a woman on a New Delhi bus has been found dead in his prison cell in India.

Ram Singh is accused of driving the vehicle on which a 23-year-old student was raped and assaulted by a group of six men in December.

Singh, along with four other men, was on trial on rape, murder and abduction charges.

The five men were being held in separate cells in separate buildings at the prison. If convicted, the men could face the death penalty.

The sixth accused is being tried and jailed separately because he is a juvenile.

Tihar jail is India's highest security prison and officials there are likely to face tough questions about how such an incident could have occurred.

India's Home Minister Sushil Kumar Shinde called the incident a "major lapse" in security and said an inquiry had been launched.

Prison authorities said it was suicide, but Singh’s family and lawyer said they suspected "foul play".

Prison spokesman Sunil Gupta said Singh was found dead just before dawn in a cell he shared with three inmates.

However, Singh's lawyer, V K Anand, said his client had been composed and calm when he spoke to him in court on Friday and "didn't have any complaints".

Singh had not been on suicide watch, the lawyer added.

On 16 December, a woman and her male friend were attacked after boarding a bus as they tried to return home after watching a film, according to police.

The woman was repeatedly raped and tortured with a metal bar.

The couple were also severely beaten before being thrown onto a road.

The woman died from her injuries two weeks later in a Singapore hospital.

The brutal attack set off nationwide protests about India's treatment of women and spurred the government to hurry through a new package of laws to protect them.