The 41-year-old told The Sun he was seconds away from disaster when he took part in a challenge, which involved him falling backwards off a mountain.
Best said that the stunt required McCann to pull a cord to counter his weight, allowing him to ease towards the ground safely.
But McCann didn't pull hard enough and staff had to rescue Best urgently, saying: "If we didn’t save the cord, that thing was splitting and you were falling to your death".
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The reality TV regular and son of football legend George Best added: "In the moment you want to support your friend and say ‘it’s okay’ but, in hindsight, you look back and you go: ‘Jesus Christ, what the f***?'
"If that had gone a little bit wrong for two more seconds, I could have been falling backwards to my death.
"It was like: ‘For f***’s sake woman, please pull the cord and don’t kill me. Come on, I’m not killing you. You don’t kill me — please'.
“I think then seeing Ferne crying, I just thought ‘it’s okay'. I might have just nearly died, but you have done well learning from this experience."
Best is set to compete alongside McCann and a selection of other famous faces on the series, including Love Island winner Amber Gill, Olympic javelin thrower Fatima Whitbread and actor Jennifer Ellison.
He said taking part in the series will allow him to showcase a new part of his personality, separate from his previous career in the reality TV world.
"I’ve never been able to show that I like extreme adventure. I like climbing mountains," Best said.
He added: "My entertainment career has really been focused around dating or documentaries about alcohol and parental drinking. At the end of the day, I’ve got to pay the bills. I wasn’t left a trust fund. I wasn’t left any money."
Celebrity SAS: Who Dares Wins will begin airing on Channel 4 from Sunday 4 September.
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