One Direction, Emeli Sande, Rita Ora, Paloma Faith and Ellie Goulding are joining dozens of music stars to record the Band Aid 30 single today.
They are singing a new version of Do They Know It's Christmas? to mark the 30th anniversary of the song which was originally put together with another all-star line-up.
Chris Martin, Sam Smith, Ed Sheeran were among the other acts due to arrive at a west London recording studio.
It will be the fourth time the song has been recorded, with the most recent effort released a decade ago.
One Direction's Niall Horan arrived to loud cheers from fans gathered outside the studio.
He said: "To be involved and get invited by Sir Bob (Geldof) to do this is just insane, so it's a privilege to be here."
Asked what he hoped to achieve, he added: "Hopefully it gets to number one and it raises a lot of money for a really worthy cause."
Some of the 2014 lyrics have been changed from the 1984 version to reflect the current Ebola crisis as the song is raising money for victims of the disease.
The original words: "Where the only water flowing is the bitter sting of tears" have been replaced with "Where a kiss of love can kill you and there's death in every tear".
And the words: "Well tonight thank God it's them instead of you" have been changed to: "Well tonight we're reaching out and touching you."
The 1984 lyrics: "And there's won't be snow in Africa this Christmas time. The greatest gift they'll get this year is life. Where nothing ever grows. No rain or rivers flow. Do they know it's Christmas time at all?" have been changed to the following:
"No peace and joy this Christmas in West Africa. The only hope they'll have is being alive. Where to comfort is to fear. Where to touch is to be scared. How can they know it's Christmas time at all."
The original words: "Here's to you. Raise a glass for everyone. Here's to them underneath that burning sun. Do they know it's Christmas time at all?" have been altered to say:
"Here's to you. Raise a glass to everyone. And here's to them. And all their years to come. Let them know it's Christmas after all."
The 1984 chorus: "Feed the world. Let them know it's Christmas time again. Feed the world. Let them know it's Christmas time again. Feed the world. Let them know it's Christmas time again" has been changed to:
"Feed the world. Let them know it's Christmas time again. Feel the world. Let them know it's Christmas time again. Heal the world. Let them know it's Christmas time again."
'Against the clock' production
After its original release - to help famine relief in Ethiopia, selling 3.7 million copies - the song went on to be the launchpad for LiveAid and inspired numerous musical fundraising efforts.
The production team will be working against the clock to get the track completed in time for its first public performance during Sunday night's edition of The X Factor.
The new single, which will be available to download from 8am on Monday, is expected to raise millions to help ease West Africa's Ebola crisis.
Geldof, who co-wrote the song with Midge Ure, said he had been inspired to re-record the track because of the "phenomenal bravery of the NHS doctors and nurses who volunteered" to help the fight against the virus.
Geldof has said of the track: "It really doesn't matter if you don't like this song. It really doesn't matter if you don't like the artists, it really doesn't matter if it turns out to be a lousy recording - what you have to do is buy this thing."
Other acts lined up to play on it include U2's Bono - who appeared on the first version in 1984 - Paloma Faith, Emeli Sande, Elbow, Bastille and Jessie Ware.
Earlier in the week, Geldof said there are more "giants" of rock and roll to come, including David Bowie, who has not been confirmed.
Paul Epworth, who won an Oscar with Adele for the James Bond theme song Skyfall, is involved in updating the track.
