Album tracks, B-sides and golden oldies could enter the top 40 when chart rules are changed on 1 January.
Wham's Last Christmas is number 21 in the download charts
The shake-up means that any download will count towards the top 75, even if it is not available on the high street.
Test charts show that Christmas songs by Wham and Mariah Carey would have been in this week's festive top 40 under the new rules.
CD singles will also be allowed extra tracks and extended running times to help them compete with downloads.
"This new ruling changes the nature of a single and puts the consumer in the driving seat," said Steve Redmond, director of the UK Official Charts Company.
"Literally any track can be a hit - as long as it sells enough."
High street opposition
Previously, downloads only counted towards the chart in the week before a song was released on CD, and until two weeks after the CD was deleted.
This meant that record shops could avoid having blank spaces in their displays.
The revamped rules were first revealed on the BBC news website in September, when it was reported that high street retailers had dropped their opposition to the inclusion of download-only tracks.
Reacting to the official announcement of the changes on Thursday, Virgin Megastores called the new chart "a positive move forward".
Several retailers contacted by the BBC indicated they would fill blank spaces in Top 40 displays with notices indicating a song's availability in their online stores.
Downloads have helped to revitalise the UK's singles market, driving sales up from 32.3m in 2004 to 62.1m in the first 51 weeks of 2006.
US hip-hop duo Gnarls Barkley became the first act to reach number one purely through downloads in April.
Their single, Crazy, is currently the UK's most-downloaded single, with 397,000 copies sold - although X Factor winner Leona Lewis may overtake that figure this week.
X Factor winner Leona Lewis sold 100,000 downloads in two days
The current top 40 downloads includes 18 songs not eligible for the main chart, including old hits such as Wham's Last Christmas and Snow Patrol's Chasing Cars, as well as future singles by Eminem and Take That.
It is hoped that by including downloads the pattern of singles slowly climbing up the charts will return.
With digital downloads appealing to older music buyers, it is also thought that the singles chart could see a greater diversity of artists in 2007.
The first revamped top 40 will be revealed on JK and Joel's Radio 1 Chart Show on 7 January.