POP music history is expected to be made today when a single reaches number one in the charts through sales only of computer downloads.

Crazy, by the American hip-hop duo Gnarls Barkley, is tipped to top the official singles chart even though it will not be available in the shops in CD format until tomorrow.

Its chart success is seen as an important milestone for legal downloads, which are credited with reviving the market for sales of pop singles.

By midday on Friday more than 18,000 people had downloaded Crazy, 20% more than any other song last week. It costs 79p through online music shops such as iTunes and HMV.co.uk.

Martin Talbot, editor of the trade magazine Music Week, said: “It’s a big moment.”

The track is described variously as neo-soul, hip-hop and RnB. It features soulful vocals over sweeping violins and a twangy bassline.

Gnarls Barkley is made up of Brian Burton, known as Danger Mouse, who produced the hit album Demon Days by Gorillaz, and singer Thomas Calloway, known as Cee-Lo.

Crazy was released as a download on March 13, although only sales since last Sunday count towards its chart position today.

In the first 12 weeks of 2004, 6.1m singles — mainly CDs — were sold in shops compared with 227,000 downloads. In the first 12 weeks of this year, 10.5m singles have been downloaded and 3m have been sold in shops.

Steve Redmond, a spokesman for the BPI, the record companies’ organisation, said: “For a long time people have been saying the single was dead. The download is contributing to the resurgence of the singles market.”

Redmond said digitally downloadable singles could also make the charts more like they were in the 1970s and 1980s, when growing grassroots popularity could slowly propel a single up the listings over a period of time.

In recent years a single’s highest place in the charts has often been its entry position, following an expensive marketing campaign by the record company.

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