Crazy by Gnarls Barkley has been tipped to make music history this week by becoming the UK's first number one single based on download sales alone.
The enigmatic duo play the 02 Wireless Festival in June
Until this month, download sales could only count towards a chart position if it was also available to buy in stores.
But a change in the rules has enabled download sales to be considered - as long as physical copies of the single go on sale the following week.
One retailer, HMV, said it was its fastest-selling music download ever.
A spokesman for the chain said it would represent "a significant landmark in charts history" if the track hits the number one spot on Sunday.
Gnarls Barkley consists of two Americans - producer Danger Mouse (aka Brian Burton) and hip-hop artist Cee-Lo Green, aka Thomas Calloway.
Their song has been in demand since featuring in BBC TV promotions for Zane Lowe's Radio 1 show.
A spokeswoman for the Official UK Charts Company confirmed the song would be the first downloads-only chart-topper if it reached the number one position.
However, she said midweek sales figures were confidential and could not be disclosed.