Simon Cowell had doubts about Robbie Williams

Simon Cowell was worried about hiring Robbie Williams for \'The X Factor\'.

The 58-year-old music mogul admitted he was unsure whether to invite the former Take That star to join the judging panel of the talent contest because he was worried he would quickly get bored and want to quit, but he\'s been relieved to find his concerns were unfounded.

He said: "I always had that worry which is, you know, after three hours he might just go, \'I\'ve had enough\'.

"But I would say almost every day we would finish and then he\'d email me afterwards, \'Loved today, best day of my life blah, blah, blah\'.

"It\'s almost like he\'s back in a group again if that makes sense."

Though he\'s known for his own vanity, Simon insists that Robbie actually has the bigger ego.

Asked which of them has the highest opinion of themselves, he told The Sun newspaper: "Oh definitely Robbie. Oh my God yeah. I just realise why he is who he is, I mean he\'s so funny.

"He loves the adulation, he thrives off it but in a fun way, he doesn\'t hide from it, he loves it."

Robbie\'s former Take That bandmate Gary Barlow served as head judge on \'The X Factor\' from 2011 to 2013 but Simon admitted he prefers having the \'Angels\' hitmaker on the panel.

He said: "I\'ll be honest with you, I\'ve been friends with Robbie for a number of years.

"Robbie\'s a friend, so of course I\'m going to say Robbie."

Meanwhile, Robbie, 44, recently admitted he spent years being "terrified" of Simon because he turned down the chance to sign Take That in the early 1990s.

He said: "I was terrified by him and really interested in him as a person. He was an incredibly interesting guy who I was scared of! He had a chance to sign Take That and he passed on it."

Robbie and Simon are joined on this year\'s \'X Factor\' panel by the \'Candy\' singer\'s wife Ayda Field and One Direction star Louis Tomlinson.