Gnarls Barkley, the collaboration between Goodie Mob co-founder Cee-Lo and artist/producer Danger Mouse, looks set to make a significant impression on the U.K. singles chart.

Its soul-tinged single "Crazy" will be released April 3 on a physical CD in the United Kingdom by Warner Bros., but has been available as a digital download since March 13. The track is No. 1 today at Apple's iTunes Music Store U.K., ahead of Orson's "No Tomorrow," which tops the official U.K. singles chart combining physical and download sales.

As previously reported, new eligibility rules on that tally mean that digital sales of a track are now accepted one week before the commercial equivalent, so "Crazy" is likely to reach the chart next week on downloads only.

"This is all a big mystery," Danger Mouse tells Billboard.com with a laugh about why "Crazy" is blowing up. "Cee-Lo is always asking me, 'Why? Why do they like this so much?" Not that we don't like the song. We do. But the first time that song was ever sung out of his mouth is what you hear. One take."

Gnarls Barkley's debut album, "St. Elsewhere," will be released April 24 in the United Kingdom and May 3 in North America via Downtown/Atlantic. The group's first live performance is set for May 1 at the Coachella Valley Music & Arts Festival in Indio, Calif.