Songwriter Kara DioGuardi has written hit singles for the likes of Faith Hill and Céline Dion. (Fox Broadcasting/CTV)

American Idol has lost another judge with Kara DioGuardi leaving in the wake of departures by Ellen DeGeneres and Simon Cowell, leaving Randy Jackson as the only judge left.

"I felt like I won the lottery when I joined American Idol two years ago, but I feel like now is the best time to leave Idol, " DioGuardi said in a statement Friday, calling her experience as a judge on the show "amazing."

"I look forward to my next challenge, and want to thank everyone who has supported me. All the best to everyone on Season 10!"

The songwriter's exit had been rumoured for months now. The statement, issued by Fox, gave no reasons for her departure.

DioGuardi is a known quantity in the music business, writing hit songs for the likes of Céline Dion, Gwen Stefani, Faith Hill and many others.

"She has been passionate and committed to Idol over the last two seasons," said the show's creator and executive producer Simon Fuller in a statement.

"I will miss having her on the show, but I look forward to working with her in music for many years to come."

Her departure comes just as stories in Hollywood have swirled that the network had secured actress and singer Jennifer Lopez for next season and that negotiations were going well with Aerosmith frontman Steven Tyler.

The acerbic Cowell, known for his often scathing comments, was first to leave when he announced he was launching another talent show for Fox, The X Factor, based on the British series he had created.

Then in July, DeGeneres, who was brought in to replace Paula Abdul, said she was leaving because she wasn't suited for criticizing young talent.

The No. 1-rated show has seen a significant slide in viewership — it lost about five million viewers in its finale this year, which Lee DeWyze won, compared to the final in 2009, which saw Kris Allen trump Adam Lambert.

With files from The Associated Press