The singer's second studio album has been inside the top ten ever since its release in January 2011.
It has now fallen two places to number 12 in its 72nd week of release.
21 has become the fifth best-selling album in UK chart history, selling around 4.34 million copies.
However, the album is likely to re-enter the top ten eventually, as Adele is reportedly planning to re-release 21 with brand new extra tracks.
The LP has also passed the nine million sales mark in the US.
Despite the sales figures for 21, UK album sales were recently revealed to be at their lowest for 16 years.
Gary Barlow is currently number one in the album chart for a second week with Sing.
Watch the video for Adele's 'Rolling in the Deep' below: