A source told The Sun that the details had not yet been officially revealed but Mr Friedman, 32, could not resist "blabbing" about the signings.

"Brian is really excited about this year's show because there are some great artists involved. He is proud to be part of it and admires Simon's ability to attract A-listers."

Madonna, Williams, Houston and Rihanna will give personal masterclasses to some of the contestants during the finals of the show, which starts later this month.

Last year mentors included the boyband Westlife, Take That's Gary Barlow and Mark Owen and R'n'B singer Beyoncé.

The revelations show that ITV producers have decided to dig deep into their budget as they compete with other programmes for viewers.

But the programme will no longer have to compete with Strictly Come Dancing on Sunday evenings. The BBC is to abandon its Sunday night results show for an extended Saturday slot to avoid any clash with the X Factor.

Couples will compete, face the public vote and have the dance-off all in a single show as the corporation attempts to make Saturday night ''Strictly night''.

It is understood that part of the reason for the change is the desire to avoid a possible clash with the X Factor results show, which may be shown on Sundays this year.

The new series of Strictly will begin in September with two special Friday night launch shows, but will switch to Saturdays only from then on.