Kate Thornton and Louis Walsh have been replaced on The X Factor.

The new format will see four judges, including Simon Cowell and Sharon Osbourne, instead of the usual three.

Walsh, 54, will continue to manage the acts off air but will no longer be a judge.

Thornton, 34, said: “Presenting The X Factor these last three years has been fantastic and I truly believe I have taken the show as far as I can. I wish the new host every success – it will be an experience they will never forget. I’m looking forward to getting stuck into some new programmes with ITV.”

Walsh, known for his run-ins with the other judges, said: “After three incredible years, I have decided to leave The X Factor to completely focus my energy on my day job as an artist manager. I will be having the busiest year of my career with Westlife, Shayne Ward and the re-launch of Boyzone.

“I am also looking forward to continuing my work with Simon and The X Factor team, managing the stars from this year’s series.”

ITV director of comedy and entertainment Paul Jackson said: “To keep the show in its number one spot we are always looking at ways of making the format as fresh and exciting as ever and having an extra judge and additional category will give the show a whole new dimension. We have to keep evolving and I’m really excited about series four.”

A spokeswoman said no new hosts or judges had been lined up yet.

Producers have also decided to lower the age limit to 14 and to split the 14 to 25-year-old group into boys and girls to create four categories mentored by four judges.

Thornton will be signing up to new ITV projects.