More than 72,000 copies of the new Band Aid single were sold on its first day of release, according to sales figures.
The single is expected to sell up to 600,000 copies this week
A Band Aid spokesman said they were "very, very happy" with Monday's demand, which saw shops open early.
That makes it the fastest-selling single of the year so far, shifting more copies than the rest of the top 30 combined, according to HMV.
Dido, Robbie Williams and Chris Martin are on the new single, which is raising money to fight hunger in Africa.
Estimates of how many copies the 2004 version of Do They Know It's Christmas? could sell by the end of the first week range from 250,000 to 600,000.
If it sold 500,000 copies, more than £1m would be raised for charity. The CD is being sold for £3.99, with HMV, Virgin and Woolworths donating their profits.
HMV spokesman Gennaro Castaldo said: "I think the single will continue to sell steadily in large quantities over the coming weeks to nudge it towards the million mark, which would be a fantastic achievement."
The original version, which featured stars including George Michael, Phil Collins and Duran Duran, sold 750,000 copies in its first week and 3.5 million in total.
That made it the UK's biggest-selling single and it has only been surpassed by Sir Elton John's tribute to Princess Diana, Candle in the Wind '97.
The new Band Aid also features Bono reprising his original lyric from the 1984 version, plus scores of modern stars including Will Young, Jamelia, Dizzee Rascal and Sugababes.
The current top-seller of 2004 so far is Eamon's I Don't Want You Back with more than 500,000.
The last million-selling singles in the UK were by Pop Idol stars Will Young and Gareth Gates in 2002.
Meanwhile, the new Band Aid single has jumped from 33 to three on the latest TV airplay chart. But it has slipped from 14 to 22 in the radio airplay chart after the initial rush to get it on air.
Money is also being raised through internet downloads and mobile phone ringtones.
Users of the world's most popular online music store, Apple's iTunes, can also now download the track. It was previously only available in Windows Media format.