A source close to Ronaldo's agent, Jorge Mendes, told the News of the World: "Initially, the only thought was for a new deal, nobody was even contemplating Cristiano leaving because he'd been so accepted at United and was playing so well.
"When it became obvious United were reluctant to even discuss an improvement, the situation changed.
"There was a little bit of anger and disappointment among Ronaldo's people.
"Inevitably, Madrid got to hear about it and that's when the whole thing snowballed."
The Mendes source added: "It wasn't just the £300,000 a week wages that turned Cristiano's head.
"Real sold him the whole story. They talked about history, about Real being the most glamorous club in the world and how he would be the man to lead them back to Champions League glory.
"His mother Dolores was behind the move from the start and she is a very influential voice in his life.
"It took a little while, but Cristiano bought into the idea and from then there was no turning back."