The Arctic Monkeys' album could become the fastest-selling debut album in recorded chart history, after selling more than 100,000 on its first day.
Arctic Monkeys topped the singles charts for the second time on Sunday
Whatever People Say I Am, That's What I'm Not sold 118,501 copies, outselling the rest of the top 20 albums combined.
It is expected to sell over 350,000 copies by the end of the week, breaking Hear'Say's five-year-old record.
"We knew day one sales were going to be big, but nobody expected them to be this huge," said HMV's Phil Penman.
The retail chain's head of music added: "It's as if they had won the X Factor, but achieving the same kind of sales without the benefit of a massive TV audience."
The Sheffield band built up a substantial fan base via the internet, before they were signed to Domino Records in June 2005.
Their debut single I Bet You Look Good On The Dancefloor topped the charts last October.
Follow-up When The Sun Goes Down gave the band a second number one last Sunday, and current sales figures put them in line to simultaneously top the album and singles charts this weekend.
The album has already sold more in one day than the debut efforts from Kaiser Chiefs, Coldplay and Franz Ferdinand sold in their first week.
"They're well on their way to having the first million-selling album of 2006," said Mr Penman.
"If it continues to sell at this rate, there's even a danger shops could sell out by the end of the week."
Hear'Say's Popstars holds the current record for a debut album, selling 306,631 copies in its first week in 2001.
Earlier on Tuesday, the band picked up four nominations at next month's NME Awards, including best British band, new band, live band and best track.