"He fell in love with Victoria and that changed everything," Ferguson said at his press conference launching his new autobiography.
"He lost the chance to become an absolute top-dog player," Ferguson wrote in his book.
"He never attained the level of an absolute top player… He started to make decisions that rendered it hard for him to develop into a really great footballer. That was the disappointment for me.
"There was no animosity between us, just disappointment, for me. Dejection. I would look at him and think ‘What are you doing, son?'"
Ferguson also gave his side of the infamous incident in 2003, in which Ferguson kicked a boot across the changing room during a half-time rant that hit Beckham in the face, leaving him with a gash above his eye.
"He was around 12 feet from me. David swore. I moved towards him and, as I approached, I kicked a boot. It hit him right above the eye," he wrote.
"He rose to have a go at me and the players stopped him. Sit down, I said. You've let your team down, you can argue as much as you like,"
Ferguson added that he felt Beckham intentionally played up the extent of his injury in the media.
"He wore an Alice band that highlighted the damage inflicted by the boot. It was in those days that I told the board David had to go.
"The minute a Manchester United player thought he was bigger than the manager he had to go.
"David thought he was bigger than Alex Ferguson. There is no doubt about that in my mind."
Ferguson claims he is not bitter about his most famous former player, however.
“I hold no rancour towards David at all. I like him. He’s a wonderful boy," he added.
"But you should never surrender what you’re good at.
"David was the only player I managed who chose to be famous, who made it his mission to be known outside the game."