Green Day's comeback album "21st Century Breakdown" (Reprise/Warner Bros) is likely to secure the U.K. No. 1 based on just two days' sales, according to U.K. entertainment retailer HMV.

The new Green Day set went on sale today (May 15) as part of a worldwide release, rather than the normal U.K. release day of Monday. Metallica also had a Friday release with "Death Magnetic" (Vertigo/Universal) last September, although some retailers broke the embargo. It's not yet clear if "21st Century Breakdown" was available early anywhere in the U.K.

HMV estimates that Green Day will hit No. 1 in the albums chart on Sunday (May 17) with a similar two-day tally to "Death Magnetic," which scored total U.K. sales of 75,000 at all retailers in its first two days on sale.

"This is vying with the release of U2's 'No Line on the Horizon' to be the biggest rock event of the year so far," said HMV chart manager John Hirst, adding that the band is "back with a bang."

"Chances are Green Day will sell the best part of 75,000 copies [total U.K. figure] of '21st Century Breakdown' in just two days this week to guarantee the No. 1 spot and making it the fastest-selling album of 2009," he added. "We've seen huge demand at HMV - both through our stores and online at hmv.com and it's also our most popular MP3 download by far."

If they do finish ahead of albums by Maximo Park, Lady Gaga and Lily Allen on Sunday, Green Day will face a tough challenge to hold on to No. 1 the following week. Eminem's "Relapse" (Interscope) is out on May 18 and the rapper is giving the set a big U.K. push with an appearance on BBC One TV show "Friday Night With Jonathan Ross" tonight (May 15) and a session on Zane Lowe's show on national top 40 station Radio 1 on Tuesday (May 19). Green Day is in session on the show the following day.

Manic Street Preachers will also be fighting for a second U.K. No. 1 album - its last chart topper was 1998's "This Is My Truth Tell Me Yours" (Epic) - with "Journal For Plague Lovers" (Columbia) next week. The new set has had strong reviews, although mass merchants Sainsbury's, Tesco, Asda and Morrisons are insisting the album sleeve be covered by a plain slipcase, because the portrait by Jenny Saville appears to show the face of a boy covered with blood.