Gwen Stefani took weeks to complete new song Spark The Fire because she had to keep tracking down producer Pharrell Williams all over America.

Pharrell introduced the No Doubt star to a beat he was working on for her new solo album when they started out as judges on U.S. Tv talent show The Voice and they started working on the track that night.

She tells Los Angeles DJ Ryan Seacrest, "He plays me this thing and he starts singing, just in the air, and everybody in there was like, 'Ok, there's some magic going on right now'. I just hugged him, I almost started crying. You could tell he hit on it.

"I videotaped us doing (it), it was so exciting, but we didn't finish it. We stayed up all night, I had to be back on-set at 7am. Then he was gone... I had to go find him, so when I was in New York for my fashion show, had a crazy week, and same thing again, met him in studio in the middle of night, tried to finish, still didn't finish it."

The two stars finally completed the track weeks later in Miami, Florida and Stefani can't wait for fans to hear it: "It literally is the song that explains everything that's going on in my life right now in this surreal way."