Image caption The series opener was watched by 11.1m people

X Factor bosses have admitted using vocal enhancement processes on the ITV talent show.

Fans had claimed after the opening show that "auto-tuning" had been used to improve some contestants' voices.

It is used in recording studios to improve performances by correcting pitch and disguising off-key mistakes.

An X Factor spokesman said post-production work was necessary on the show due to the number of microphones used during filming.

ANALYSIS - Daniel Griffiths, editor, Future Music Gamu Nhengu - Walking On Sunshine That was pretty clean and then, half-way through, when she got going on the second verse, there's a 10-second chunk where it's really, really sharp and there's an autotune moment. Somebody took the time to go, "I'm going to leave this bit dry, and then I'm going to fix that bit". But they did a bad job of fixing it. G&S - Don't Stop Believing Poor fella, he doesn't get a drop of it, and she's set to maximum. The whole of her performance has got the old robot effect over it. They obviously ramped up the fact that they wanted her to be brilliant and him to be rubbish for the comedy. As far as manufacturing that scenario goes, using auto-tune makes sense. Shirleena Johnson - Mercy The whole point of that was she was awful and couldn't sing. And yet that was one of the most pronounced auto-tune moments. She was so far off that you could hear it snapping.

"The judges make their decisions at the auditions stage based on what they hear on the day, live in the arena," the spokesman added.

"The footage and sound is then edited and dubbed into a finished programme, to deliver the most entertaining experience possible for viewers.

"When it gets to the live shows, it will be all live."

Fans took to The X Factor's internet forums after the show to voice their concerns.

The focus of the debate surrounded 18-year-old contestant Gamu Nhengu, who sang Katrina and the Waves' hit Walking On Sunshine.

After hearing her effort, judge Simon Cowell told her she was "really talented" before she was unanimously voted through to the next round.

But viewer Smstrat91 said: "Cannot believe they autotuned her, listen to the words 'door' and 'telephone' and it's very clear. She's so good as well, but obviously the producers of X Factor didn't think she was good enough."

The first episode of this year's series was watched by 11.1 million people, up from last year's figure of 9.9 million.