O'Leary will present the next two series of the X Factor

Earlier this month it was announced that Kate Thornton, who hosted the show for three years, would be moving on to work on other projects.

O'Leary, best known for presenting Channel 4's Big Brother's Little Brother, said he was "flattered" to have been offered the job.

Auditions for anyone wanting to take part in the next series begin in May.

'Warm and clever'

"I'm really looking forward to working with Simon and the team on the kind of Saturday night entertainment show I grew up watching," added O'Leary.

Duncan Gray, ITV's Controller of Entertainment said: "Dermot is a star. He's warm, clever and funny and he was our number one choice for the job. We're thrilled to have him on ITV."

As well as Thornton, Louis Walsh will also be absent from the fourth series, due to start in the summer.

X Factor winner Leona Lewis signed a US record deal last month

It has not been revealed who will replace the Irish music mogul, but he will continue to manage acts off-screen.

For the first time, contestants aged between 14 and 16 will be allowed to enter the competition.

Leona Lewis, winner of last year's series, reached number one with her single, A Moment Like This.

Runner-up Raymond Quinn recently got to number one in the charts with his debut album, Doing It My Way.