The drop was even bigger -- about 27 percent -- among viewers 18 to 49-year-olds, the audience group most coveted by advertisers, Nielsen figures showed.

But despite the fall-off, which Fox television executives said they had expected, the two-hour "American Idol" season premiere was still the most-watched show by a huge margin on U.S. television Wednesday night.

The 21.6 million viewers on Wednesday, compared to 26.2 million who watched the 2011 season premiere when new judges Steven Tyler and Jennifer Lopez made their first appearance.

Fox television chairman Kevin Reilly said last week that the network was expecting the audience for "American Idol" to be down this season because of the show's age.

But media watchers say "Idol" -- the most-watched show on U.S. television for seven years -- faces the stiffest competition ever in 2012 from rival singing contests "The Voice" on NBC, and the debut season of "The X Factor" on Fox, which ended just one month ago.

(Reporting By Jill Serjeant; Editing by Bob Tourtellotte)

• A Permanent Economic Emergency