Real Madrid star Cristiano Ronaldo still holds a passion for former club Manchester United, revealing a promise from Alex Ferguson that he would play regularly was key to his move to Old Trafford.

Ronaldo arrived in Manchester from Sporting CP as an 18-year-old in 2003 and lived up to his remarkable potential in a trophy-laden six-year spell before earning a move to Madrid, where he has staked his claim as one of the greatest players of all time.

The Portugal captain is frequently linked with a return to United and revealed his love for the club was partly engendered by former boss Ferguson as well as agent Jorge Mendes.

"I feel passionately about Manchester United. I've said that many times," he said at a news conference for his new film entitled 'Ronaldo'.

"What Jorge Mendes did for me at 18, bringing me to one of the best clubs in the world under Alex Ferguson, was like a dream to me. 

"Playing for Sporting Lisbon and signing for Manchester United was like... 'Wow, how did this happen?' It was amazing.

"When Ferguson wanted me and said I'd play 50 per cent of the games that was unbelievable.

"I took that decision because Jorge helped me. The last word from Ferguson was key also."

Ronaldo's film explores, among other things, his quest for excellence as well as his apparent rivalry with Barcelona star Lionel Messi, which the 30-year-old has frequently played down.

"I don't have rivals in football," Ronaldo added, while he was also asked about whether he regretted the film project.

"[Did the film reveal] too much? Maybe, but I just had fun.

"I don't regret anything. I feel unbelievable. When you do something people will find reason to criticise you, but it's normal.

"Since I was a young boy I always felt special. I saw the people looked at me with different eyes. I feel very comfortable with the cameras, with the media."