Gnarls Barkley's new single Crazy has has been tipped for number one this week based solely on downloads.

Due to a change in chart rules, download sales can now be considered if the single is due out in stores a week later. Previously it was only possible to count download sales if the single was already available.

According to HMV, the single is the fastest-selling music download ever, and it would be "a significant landmark in charts history" if it achieves number one.