Another serious crime has been alleged against the deaf signer in South Africa whose nonsense sign interpretation caused a debacle at Nelson Mandela's memorial event.

According to news agency AP, Thamsanqa Jantjie, 34, was a member of a gang which burned alive two men by setting them on fire in 2003. The allegation was made by close relations of Jantjie, including his cousin and friends.

He was allegedly part of a group thatb put tyres around the necks of two men and then set them alight after the pair were caught with a stolen television set. It was a form of vigilante justice known as "necklacing".

Jantjie told local media: "It was a community thing, what you call mob justice and I was also there."

It appears that Jantjie did not stand trial in 2006 for the deaths - unlike his alleged fellow conspirators - because he was deemed unfit because of mental health problems.

The new claims will raise further questions about the security vetting procedures for the memorial event attended world leaders. Jantjie stood only feet away from many of them, including the most heavily protected person on the planet - US president Barack Obama.

Jantjie has endured a torrent of allegations since springing to public attention for his inept performance onstage in Soweto, which left deaf viewers furious. It emerged in the aftermath of the fiasco that Jantjie had been arrested five times for offences including murder and rape before 2003.

He spent more than a year in a mental institution for health problems.

The allegation that he took part in vigilante justice will count as another blow to his reputation.

He claimed that his signing skills collapsed onstage due to an attack of schizophrenia which left him "seeing angels". "I tried to control myself and not show the world what was going on. I am very sorry. It's the situation I found myself in," he said.

Jantjie insisted that he was highly skilled at the craft: "I think I have been a champion of sign language."