The four-time gold medallist was almost refused entry back into the UK after competing for Team GB when he was 16.
Mo – who has revealed he was trafficked to the UK as a child – was in a youth team competing against Poland and Germany in Neubrandenburg, north of Berlin, in August 1999.
But coach Mark Gregory has revealed how a customs official at Heathrow wanted to send Mo back to Germany, where he would have become Herr Farah.
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Mark explains: “The team manager asked me to take an athlete through ‘foreign entry’ as he didn’t have a valid passport. That was Mo. What he had was a document from the government passport office saying he would get a passport.
“At passport control the official looked at the document.
“He returned it to me and said, ‘Sorry, that’s not enough, he can’t come through’. They said he had to return to where he had come from.”
Mark got into a row with the official. He says: “We were stood there in our Team GB tracksuits. He was looking at this document, saying, ‘No chance’.
“I told them he had just represented GB and won his race. But he was saying, ‘It doesn’t matter –it’s not legal’.
“The customs officer said access has to be denied and he must return to where he has come from. I raised my voice and gesticulated, saying it was outrageous.”
After a stand-off the officer looked at the papers again and reluctantly let them through.
Mark, the 4 x400 metre relay coach, insisted on speaking to someone more senior.
He adds: “He looked Mo up and down, put the paper on the desk, then stamped it and curtly returned it to me. We hurried through, in case he changed his mind.
“Had I not fought Mo’s case and accepted the official’s orders, he could well have been stopped entry and sent back.
“I know that DLV (Deutscher Leichtathletik-Verb and, the German Athletics Association) would have welcomed him, looked after him and his family with a house, income, etc and made him one of theirs.
“You can, of course, speculate that Mo could well have then won four gold medals for Germany and not GB.”
Dad-of-four Mo, 39, revealed in a BBC documentary that he was brought to the UK illegally at the age of nine and forced to work as a domestic servant.
He was named Mohamed Farah by those who flew him over from Djibouti, East Africa.
His real name is Hussein Abdi Kahin and he was sent to Djibouti by his mother Aisha after his father died in a civil war in neighbouring Somaliland.
Aisha had no idea he was then trafficked to the UK and it was 10 years before they re-established contact – which was movingly retold on TV.
Mo said he had never met the woman who sent him here then made him look after another family’s children.
Mark, who has seen the show, said: “If they had taken Mo to one side, to fully check his background , who knows where it may have led. Could the truth about being trafficked been found out?
“At the time Mo was a really talented kid, a rough diamond. There were high hopes for him, but no one could have predicted the heights he reached. We can joke about him representing Germany, but there are much more serious issues involved.”
Mark met Mo again at an event in London in 2006 – and Mo thanked him for getting him back into the UK.
A spokesperson for Mo declined to comment. British Athletics was approached for comment.