The new Band Aid single is "flying out of the shelves" on its first day of release, its record company has said.
The single is expected to sell 500,000 copies in its first week
The 2004 version of Do They Know It's Christmas? is expected to sell about 500,000 copies in its first week, which would see fans spend almost £2m.
On Monday, Mercury Records said it was "very early days" but sales were going "absolutely fantastically well".
Dido, Robbie Williams and Chris Martin are on the new single, which is raising money to fight hunger in Africa.
Many record shops across the UK opened an hour earlier than normal, at 0800 GMT, to allow people to buy copies on their way to work.
By 1100 GMT, Mercury said early reports from record shops were positive.
Joss Stone was among those at the Band Aid recording session
"We've had one retailer in who's said it's flying out of the shelves, so they're particularly happy," product manager Shane Murray said.
Supermarkets are expected to do a healthy trade as people buy lunch or groceries, while record shops like HMV and Virgin are likely to attract Christmas shoppers, he added.
Cousins Kirsty and Sarah Burgess, both 21, were first in line when HMV's flagship Oxford Street store opened.
"I thought I would come along because it's a great cause," Kirsty said. "I was about one when the first single was released but I've heard it about 100 times.
Hopefully this one will raise lots of money too."
Nathan Denham, 20, was also waiting. "It's a big thing," he said. "Especially after the last one. I want to get it early. I'll probably buy a few copies."
The original version of the charity track, which featured 1980s stars including George Michael, Phil Collins and Duran Duran, sold 750,000 copies in its first week and 3.5 million in total.
The new version also features Bono reprising his original lyric from the 1984 version, plus scores of modern stars including Will Young, Jamelia, Dizzee Rascal and Sugababes.
Virgin Megastores spokesman Simon Dornan said sales were "phenomenal" and it was outselling the nearest new release by 23 to one.
"Everyone knew it was going to be big," he said.
"There can't be anyone in the country that doesn't know it's out today and like 20 years ago, a lot of people will be buying it for themselves and probably another one or two as presents."
HMV spokesman Gennaro Castaldo said: "It is clearly going to be the number one bestseller.
"I don't think it will match the last one but it will certainly be a million-seller and raise the profile of the problems in Africa."
The last million-selling singles in the UK were by Pop Idol stars Will Young and Gareth Gates in 2002.
Mr Castaldo said the chain had received a telephone order for 130 copies which, at £3.99 each, comes to £518.70.
HMV has joined Virgin and Woolworths to donate their profits, while the government is to refund the VAT paid on the single and recently-released Live Aid DVD.