Louis Walsh will not be returning to the X Factor as a judge to allow the talent show to usher in a "new era".
The show's longest-serving judge has appeared in every series of the music show, apart from one, since it started in 2004.
He will remain as head judge on Ireland's Got Talent, production company Thames confirmed.
Walsh said: "I've had a fantastic 13 years on The X Factor but the show needs a change and I'm ready to leave.
"I'm looking forward to another series of Ireland's Got Talent, going back to my day job as a manager, writing my memoirs and seeing the world."
Sharon Osborne will have a smaller role in the show when it returns for its 15th series later this year.
She will only judge the live episodes, skipping the pre-recorded audition process.
"I can't wait to get back to the judges' desk. I love the new format," she said.
Simon Cowell, who created the show but has only been a judge in six series, will return for the new series with three new judges, who will be announced soon.
Amelia Brown, managing director of Thames, said: "Louis has been a huge part of The X Factor's success since it launched and remains a good friend of the show.
"His contribution cannot be overstated. We would like to thank him for all he has given to the show over the years and look forward to continuing to work with him in the future.
"We are very excited Sharon is taking on a new role on the show and we can't wait to see her as a judge on the live episodes.
"This is the start of a new era for The X Factor. I am really excited about all the ideas we have for the coming series including, of course, the new judging panel."
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Judges' auditions for The X Factor 2018 will take place at Wembley Arena in London.
More than 30,000 contestants have already entered the series following open auditions across the country.
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