Crazy by Gnarls Barkley has made pop history as the UK's first number one song based on download sales alone.
Until this month, download sales could only count towards a chart position if the song could also be bought in shops.
But under new rules, downloads can be counted as long as physical copies go on sale the following week.
The song, used in ads for Zane Lowe's Radio 1 show, has been downloaded more than 31,000 times on all internet sites, a spokesman for HMV said.
Come of age
The song, which is not available in shops until Monday, knocked last week's top track So Sick, by Ne-Yo, off the top spot.
Gennaro Castaldo, spokesman for retailer HMV, said: "This not only represents a watershed in how the charts are compiled, but shows that legal downloads have come of age."
He said if physical copies fly off the shelves at the same rate it could vie for a place as the year's biggest seller.
"It is an outstanding single so I don't think we are going to see this happening with a song every week," he added.
"It probably will be one of the summer's hits."
New entry
The duo behind Gnarls Barkley consists of US producer Danger Mouse (real name Brian Burton) and hip-hop artist Cee-Lo.
In 2004 Danger Mouse remixed the Beatles' White Album and rapper Jay Z's Black Album - producing The Grey Album.
Gnarls Barkley are set to play the 02 Wireless Festival this June in Leeds and London.
Other entries in the top 10 include Morrissey's new single You Have Killed Me, and The Streets, whose new single When You Wasn't Famous reached number eight.
In the album chart, Embrace continued their recent resurgence with their new album This New Day reaching the top spot as a new entry.
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