With sales of 43,556 last week, the album - which previously topped the list first for 11 weeks, then for a further five - climbs 2-1, swapping places with Beyonce's 4 (32,438 sales), which topped the list for the last two weeks.
In 25 weeks on the chart, 21 has spent 17 weeks at number one, the rest at number two. Although its sales last week edged up by 27 over the prior frame, 21 sold fewer copies last week than any number one since The Script's Science & Faith 40 weeks ago.
It surpasses The Singles 1969-1973 by The Carpenters to secure the longest run in the top two since Simon & Garfunkel's Bridge Over Troubled Water put together a run of 29 straight weeks in the top two in 1970. Bridge Over Troubled Water also has the distinction of being the artist album to spend most weeks at number one, amassing 33 weeks at the summit in eight separate runs in 1970 and 1971 - a record which may prove beyond even 21.
Adele's 2008 debut album 19 rebounds 4-3 (18,435 sales) on its 27th straight week in the Top 10, and completes the first ever top three made up entirely of numeric titles.
Of 28 chart weeks thus far in 2011, female solo artists have held sway for 24, with the only interruption to their superiority coming with single-week reigns from the latest albums by Bruno Mars, Foo Fighters, Arctic Monkeys and Take That. With another 24 charts to go in 2011, they have already beaten the previous record for most weeks at number one by female solo artists in a calendar year, set in 2003, when albums by Avril Lavigne, Kelly Rowland, Norah Jones, Madonna, Beyonce, Eva Cassidy and Dido racked up 23 weeks at the apex.
Adele is joined in the Top 10 this week by XL Recordings labelmates The Horrors, whose third album Skying secures this week's highest debut honours, arriving at number five (13,924 sales). It far exceeds the band's debut album Strange House, which got to number 37 in 2007, and follow-up Primary Colours (number 25, 2009). Still Life - the first single from Skying - is the last of four debuts on the Top 75 singles list at number 63 (4,224 sales).
Put together by the Nickleodeon channel in 2009 to star in its series of the same name, Big Time Rush reached number three in the US with their debut album BTR earlier this year. Helped by £4.99 pricing at Amazon and Tesco, the album now debuts here at number 14 (7,742 sales) for the quartet whose average age is 21.
Two other albums debut inside the Top 40: Californian rock band Incubus recover from the number 52 peak their last album Light Grenades reached in 2006 to make the Top 20 for the third time, debuting at number 20 (5,316 sales) with seventh studio album, If Not Now, When; and Ricky Martin debuts at number 24 (5,083 sales) with Greatest Hits. It is Martin's fifth chart album since his self-titled 1999 debut, and eclipses the number 42 peak of his previous compilation, The Best Of, which has sold 177,064 copies since its 2001 release.
Joe McElderry won the second series of Popstar To Operastar on ITV eight days ago, beating Bucks Fizz star Cheryl Baker in the final. It caused a 127.60% uplift in sales of McElderry's debut album - from 76 to 173 - which had its best week in 21, raising its career tally to 96,885. Guest Andrea Bocelli's 2007 compilation Vivere: The Best Of fares better, sprinting 173-19 (5,368 sales), to re-enter the Top 40 for the first time since March 2008. The album, which peaked at number four, has sold 782,018 copies.
Meanwhile, The Vaccines' debut album, What Do You Expect From The Vaccines?, is also resurgent, jumping 36-21 to achieve its highest chart placing for 13 weeks. Selling 5,149 copies last week, the album has sold 123,116 copies since its release 18 weeks ago.
As the lacklustre figures above suggest, overall album sales - down 5.05% week-on-week to 1,534,428 - reach a new 21st-century low for the second time this year. In fact, they are at their lowest level since week 18 of 1999 - 636 weeks ago, when they totalled 1,529,035 - and 18.80% below same-week 2010 sales of 1,889,629.