Sir Mo Farah has revealed how he was bought into the UK illegally under the name of another child.
Titled The Real Mo Farah, the documentary sees the four-time Olympic champion revealing that, "the truth is I’m not who you think I am”.
He adds that he needs to tell his real story “whatever the cost”.
The 39-year-old father-of-four, said: “Most people know me as Mo Farah, but it’s not my name or it’s not the reality.
Mo Farah and his son Hussein
Martin Rickett
“The real story is I was born in Somaliland, north of Somalia, as Hussein Abdi Kahin. Despite what I’ve said in the past, my parents never lived in the UK.
“When I was four my dad was killed in the civil war, you know as a family we were torn apart.
“I was separated from my mother, and I was brought into the UK illegally under the name of another child called Mohamed Farah.”
Sir Mo became the first British track and field athlete to win four Olympic gold medals, becoming a nationwide sporting legend.
In the BBC documentary, the athlete thanked his children for inspiring him to be truthful about his past.
Sir Mo Farah (right) and wife Tania Nell with the Outstanding Contribution to Sport Award at the BT Sport Industry Awards 2018
Ian West
He added: "Family means everything to me and, you know, as a parent, you always teach your kids to be honest, but I feel like I’ve always had that private thing where I could never be me and tell what’s really happened.
“I’ve been keeping it for so long, it’s been difficult because you don’t want to face it and often my kids ask questions, ‘Dad, how come this?’ And you’ve always got an answer for everything, but you haven’t got an answer for that.
“That’s the main reason in telling my story because I want to feel normal and… don’t feel like you’re holding on to something.”
The Olympic athlete's wife, Tania Nell, stated how in the year leading up to their wedding in 2010, she realised "there was lots of missing pieces to his story" but she eventually "wore him down with questioning" and he unveiled the truth.
Great Britain's Mo Farah celebrates with his gold medals after winning the Men's 5000m and 10,000m at the Rio Olympic games
Martin Rickett
During the documentary, Sir Mo outlines his journey to the UK, stating how he thought he was going to Europe to live with relatives, recalling going through a UK passport check under the facade of Mohamed at the age of nine.
He said: “I had all the contact details for my relative and once we got to her house, the lady took it off me and right in front of me ripped them up and put it in the bin and at that moment I knew I was in trouble.”
The athlete is videoed travelling back to his family home in Hounslow, recalling "not great memories" where he was deemed an outsider and not treated as a member of the family.
He said: “If I wanted food in my mouth my job was to look after those kids, shower them, cook for them, clean for them, and she said, ‘If you ever want to see your family again, don’t say anything. If you say anything, they will take you away’.
“So she told you don’t talk about anything otherwise I was in big trouble and I guess for me the only things that I could do, in my control, was to run away from this was get out and run.”
The 39-year-old eventually opened up to his PE teacher Alan Watkinson, explaining the truth and moved to live with his friend’s mum, Kinsi, who “really took great care” of him and he ended up staying for seven years.
His PE teacher applied for his British citizenship which he described as a "long process" and then on July 25, 2000, Farah was recognised as a British Citizen.
Sir Mo, who named his son Hussein after his real name, said: “I often think about the other Mohamed Farah, the boy whose place I took on that plane and I really hope he’s OK.
“Wherever he is, I carry his name and that could cause problems now for me and my family.
“The important thing is for me to just be able to look, this is what’s happened and just being honest, really.”
In the documentary, a barrister informs the athlete that, despite being trafficked into the country as a small child and telling the relevant authorities the truth, there is still a “real risk” his British nationality could be taken away as it was obtained by misrepresentations.
The Real Mo Farah will air at 6am on BBC iPlayer and 9pm on BBC One on July 13.