Olympian Mo Farah said he came from Djibouti aged eight or nine with a woman he had never met, was given a false identity, and then made to look after another family's children.
In fact, he said, his father was killed in civil unrest in Somalia when Farah was aged four and his mother, Aisha, and two brothers live in the breakaway state of Somaliland.
"The truth is I'm not who you think I am," Farah said in the documentary, explaining that his mother wanted him far removed from Somalia's civil wars.
He said his children had encouraged him to tell the truth about his past.
"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 admission could have raised questions about Farah's UK citizenship, but the interior ministry said he was in the clear.
"No action whatsoever will be taken against Sir Mo and to suggest otherwise is wrong," a Home Office spokesperson told AFP.
The ministry's guidance absolves children of blame if parents or guardians are later found to have obtained their immigration status under false pretenses.
Popularly known as "Sir Mo" after he was knighted by Queen Elizabeth II in 2017, Farah completed the 5,000m and 10,000m double at both the London 2012 and Rio 2016 Olympics.
The London Games in particular catapulted him to stardom in Britain. Finance minister and Conservative leadership candidate Nadhim Zahawi said Farah remained "truly inspirational".
"All I can say is I salute Mo Farah," he said.