OJ Simpson's best friend and lawyer knew he was guilty of killing his estranged wife and her lover, Caitlyn Jenner has reportedly claimed in her forthcoming autobiography.

Robert Kardashian, who passed away in 2003, made the confession during a car journey in the late 1990s, the TV personality writes in Secrets of My Life, which will be published later this month.

His admission came after he worked on the legal team which helped clear the American football star of the murders of Nicole Brown Simpson and Ronald Goldman in 1995, a case that made headlines around the world.

The pair were found stabbed to death on the driveway of her home the previous year. No one has ever been convicted for the slayings.

“I would’ve been okay with it if they had gotten him in the first trial,” Mr Kardashian said, according Jenner.

She added: “The implication was obvious: that he believed OJ was guilty”.

Excerpts from the memoir have been obtained by the Radar celebrity news website.

Although he was cleared in the criminal trial Simpson was forced to pay $33.5 million (£26.8 million) to the families of his ex-wife and Mr Goldman after losing a 1997 civil trial.

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Jenner, a retired Olympic gold medal-winning decathlete, underwent gender reassignment surgery in 2015.

Previously known as Bruce Jenner, she was married to Mr Kardashian's ex-wife Kris (then Kardashian) in 1991, when she also became a step parent to Kourtney, Kim, Khloé, who along with their two children Kendall and Kylie, went on to become world famous reality TV stars.

The pair were wed just a month after Kris split from Mr Kardashian and Jenner claims that jealousy about their relationship drove him to represent Simpson.

​Kris Jenner and the murder victim Nicole Brown Simpson were close friends, and the memoir suggests he wanted to hurt his ex-wife.

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“I wonder if it was his way of saying to her what I think she was saying to him when she married me: a big f*ck you,” the former Olympic athlete writes.

The book also claims the trial was highly divisive for the Kardashian family, as Kim and Kourtney sided with their father and believed in Simpson’s innocence.

Simpson is currently serving 33 years in a Nevada prison for kidnapping and armed robbery.