Simon Cowell's biggest regret is failing to sign the 'Spice Girls' to his record label.

The 54-year-old music mogul admitted that he was ''hurt'' when the platinum-selling girl band - made up of Victoria Beckham, Mel C, Mel B, Emma Bunton and Geri Halliwell - found worldwide success after choosing a rival record label and has regretted missing out on the chance to work with the 'Wannabe' singers ever since.

In an interview with Jay Leno on 'The Tonight Show' yesterday (9.12.12) Simon said: ''What can I admit to? Spice Girls was probably my biggest mistake. I knew they were going to big and I offered them a deal but I was just too late on the deal and they didn't want to sign to me, that kind of hurt at the time.

Simon also admitted he came close to missing out on signing Irish boy band Westlife - who went on to have fourteen UK number one hits - because he thought they were ''ugly'', but they went to great lengths to disguise themselves and he took them on after a second audition.

He said: ''I quickly put together another group. Well another group auditioned for me. They were called Westlife. When they first came along they were the ugliest boyband in the world. So seriously you couldn't get uglier. I said to the manager at the time, 'Forget it.' Then he re-auditioned but one of the ugly ones dyed his hair blonde for the second time I saw them and he got back into the group.

''I didn't recognise him but it all worked out fine. They sold about 50 million albums or something so it all worked out well enough.''