The BBC News website's entertainment reporter Ian Youngs is inside the studio where the new Band Aid single is being recorded on Sunday.
Sir Paul McCartney, Coldplay's Chris Martin and The Darkness' Justin Hawkins are among those taking part.
Updates on the stars and the session will appear throughout the day. Click more for full details...
AIR STUDIOS, LONDON :: 0845 GMT
The great Band Aid bandwagon is already in full motion, with Sir George Martin's recording studio in Hampstead, north London, a hive of activity. There is already a gaggle of fans and photographers outside the impressive converted church.
We are not allowed into the studio because Coldplay's Chris Martin is already in there recording something. Scruffy and blending in with all the beardy studio blokes, he wanders out into the foyer after a while and chats to a few people.
It transpires that quite a lot of the song has already been recorded - the music done on Friday and many of the solo lines done on Saturday.
All the talk is of Dizzee Rascal's contribution - he has written rap lines that act as responses to lines sung by The Darkness' Justin Hawkins.
AIR STUDIOS, LONDON :: 0920 GMT
I am called into an office to listen to a rough version of the parts of the song that have been recorded already.
Coldplay's Chris Martin has done the opening verse, originally recorded by Paul Young. That will definitely be in the finished version - but everything else is up for grabs.
In the rough cut, Dido has done Boy George's part, Sugababes have taken Simon Le Bon's place and The Darkness' Justin Hawkins sings Bono's famous line: "Well, tonight thank God it's them instead of you," with a typical falsetto flourish at the end.
Bono has also re-recorded this part and sent it down the phone line from Ireland, where it was his dad's 90th birthday.
Dizzee Rascal's section is startling, and brings the song firmly into the 21st Century. It works fantastically. The Darkness' elaborate guitar work plays over the part where the chorus will come in later.AIR STUDIOS, LONDON :: 1015 GMT
Midge Ure, who produced the 1984 record, comes into my booth to remind us why it is all happening again, 20 years later. He recently went back to Ethiopia and said the situation there has improved - but there are still lots of problems in the continent that need solving.
He also hopes that as well as raising money, the new song will put pressure on politicians as the UK hosts the G8 summit next year.
As for the song, he says Dizzee Rascal was "brilliant".
"He came in and did this little rap thing in the middle of the song," he says. "He did two lines - one take each - and when he realised the gap was actually smaller than what he'd written, he went off for 10 minutes and came back with this whole new line.
"He came up with this fantastic line, something about 'help the helpless', which was fabulous."
As for the wrangle over Bono's line, he says lots of people have recorded it but no decisions had been made. They may even use Bono's original vocals.
"It's got to be the one that works the best, the one that's got that hair-standing-on-end factor.
"You've got to stand back and listen to it all and think what's the best one? What's the one that grabs that moment?"
AIR STUDIOS, LONDON :: 1130 GMT
Outside, a media circus has sprung up with a line of TV cameras leading up to the studio doors.
Jamelia is the next star to arrive, with her daughter. She comes into the press booth and grabs a nearby box of cookies before hiding it under the table when a photographer starts taking pictures.
She will sing lines with Will Young - she does not know which yet - and it was "such a huge thing to be involved in".
"Of course, we can't stress any more it's for a fantastic cause and something I truly believe in," she said, revealing she would be travelling to Rwanda in January.
"I'm using my popularity to create awareness for something that's so real, it's still here. It's very sad that we're living in a world where we want for nothing but not too far away, there are people in 100% poverty."
The atmosphere was "a lot more relaxed than I expected", she said, adding she got "the biggest telling off ever" from her mum after referring to Band Aid as "that song".
Rom-com writer Richard Curtis is outside and Rita Gilligan MBE, a waitress from the Hard Rock Café, who are providing the catering, also pops in.
She would serve 200 meals on Sunday, she said. "We'll feed them all. Feed the world, as they say."
AIR STUDIOS, LONDON :: 1215 GMT
The stars are coming thick and fast, with Joss Stone, Travis frontman Fran Healy, Snow Patrol's Gary Lightbody and the man who started it all, Bob Geldof, all arriving - most wrapped up to protect against the cold.
But The Darkness' Justin Hawkins swans past the TV cameras with shirt open and chest on show plus leather trousers, only stopping to take part in a photogenic embrace with Geldof.
Inside, Jamelia and Will Young emerge from a smaller studio where they have been recording their two lines. "It took 10 minutes, so it was alright," Jamelia says.
AIR STUDIOS, LONDON :: 1300 GMT
Busted, Keane and Rachel Stevens are the latest to arrive. At one point, the cast list in the press booth includes Keane singer Tom Chaplin, Katie Melua, Busted and Feeder frontman Grant Nicholas.
Chaplin laid down his lines on Saturday - the song's first refrains of "Feed the World", he says. "It's changed a little bit from the original," he says.
"It's a slightly different take on it. I'm not even sure it was in the original. I listened to the original single a few times to try and learn how it went."
Busted, meanwhile, say the day feels a bit like the first day at school.
"Everyone's like 'ooh, should I talk to anyone?'" singer Charlie Simpson said. "I'm glad we're here as a band because a lot of people are here as a band I think and we can just talk to each other."
Matt Jay said he thought the charitable message would reach their young fans.
"I think some of our fans it will really hit home for them, and if not, I think us being involved in it will shed some light on it for them."
James Bourne added: "Charity is a term pretty much everyone understands - whether you're four or five years old, everyone knows it's for a good cause."
Others milling around include Bob Geldof's daughter Pixie, wearing the "Feed the World" T-shirt her dad wore at the original recording session 20 years ago.
Outside, the scores of photographers, fans and police are causing havoc for residents of this leafy Hampstead street. The heavy security has guarded the building, but two gatecrashers did make it in.
Two stray dogs got into the studio without anyone noticing before settling down in the corner of the hall where the finale will be recorded.
AIR STUDIOS, LONDON :: 1345 GMT
The artists are told the football team-style group photo is going to happen soon. Two rows of chairs have been set up in the hall, each with a sticker with a different artist's name on it.
In the hall and foyer, artists are tentatively mingling - The Darkness talking to Travis, Ms Dynamite and Lemar, Natasha Bedingfield sharing a chat with Shaznay Lewis and Estelle.
Everyone seems to be behaving, with numerous children and managers also milling around.
In the Hard Rock Cafe section of the studio, I grab an egg sandwich and put a donation into two metal buckets that have been put out to raise more money for the Band Aid Trust.
AIR STUDIOS, LONDON :: 1400 GMT
Bob Geldof gathers the artists to remind them this is not just another jolly.
In Band Aid 20, you're not just keeping hundreds of thousands of people going with the proceeds," he told the huddle.
"This year, when people buy your record, they're making a political statement.
"What you're doing this morning and giving up your Sunday is that you are making that political demand. Next year, we must keep that pressure up."
To press home his point, he played a video of the Ethiopian famine from 20 years ago, bringing many of these stars back down to earth with a bump.
As it ended, he introduced a young Ethiopian woman who was featured on the video as a malnourished child.
"This is proof that Band Aid and Live Aid work. You're part of it now. You're part of the gang."
There were few dry eyes in the hall, with the likes of Joss Stone, Ms Dynamites and Sugababes all overcome by the emotion.
AIR STUDIOS, LONDON :: 1415 GMT
The stars are told to assemble for the "school photo", in which almost all the participating artists line up.
It is indie blokes at the back and pop starlets at the front, with only a few absentees - Dizzee Rascal, who recorded his part on Saturday, has not come back, and celebrity bassist Sir Paul McCartney is also not here.
Despite an order for no other cameras apart from the official photographer's - he has paid a lot of money to be here - Ms Dynamite spots on the balcony taking pictures. They are nabbed by security.
AIR STUDIOS, LONDON :: 1430 GMT
Joss Stone enters the press booth, upset and angry at the emotional moment she has just witnessed.
"You can't stop me from crying, it was so embarrassing," she says.
"It's just sad. It's just so horrible and it doesn't have to be like that. I don't know why it is like that.
"We have all the food in the world and we don't even eat all of it. It makes me so mad."
She was not born when the first Band Aid happened and has never seen the 1984 video. The atmosphere this time was "nice, everyone's working together".
"This isn't about money. It's about people's lives. That's a little bit more important," she says.
After she leaves, Damon Albarn wanders in wearing an apron and carrying a tray of cups of tea, which he has been handing out outside.
He has his photo taken with Bob Geldof, who Joss Stone calls Gandalf.
AIR STUDIOS, LONDON :: 1445 GMT
Bob Geldof sits down to talk to the press. He says he showed the stars a hard-hitting video of the 1984 Ethiopian famine to remind them why they were there.
"If anything it's of more importance this time, so I just wanted [to show it to them] if there was any doubt or if they were a bit fed up that they had to give up their Sunday.
"I wanted to let them know that it works, it works at every level. I think they got it."
"You can get carried away by the event, especially if you're in a pop group. It is about something other. It's about using art and culture to move something that is a grevious sore. And it works."
AIR STUDIOS, LONDON :: 1500 GMT
The group of about 40 musicians gathers like a school choir to record the finale, each with a pair of headphones playing them the rest of the track.
They spontaneously break into refrains of "Feed the world" and "Do They Know It's Christmas?" and occasionally a very loud "1-2-3-4".
Then the musicians' resemblance to a school choir gets even more striking as they do it again with childrens' instruments - Rachel Stevens has her maracas, Fran Healy has castanets and Ms Dynamite a wood block.
Healy said the new version would be different to the 1984 original.
"This reflects the sound that's being made by the current artists," he said.
Some, like The Darkness drummer Ed Graham, did not join the chorus and he was pacing the corridor.
Will Young and Natasha Bedingfield rush out before the end, looking like they have other very important places to be.
AIR STUDIOS, LONDON :: 1550 GMT
The Hard Rock Cafe area is packed with stars getting a late lunch. The donations buckets now have about £100 of notes in them.
The choir reconvenes - this time with proper instruments. Fran Healy is on one of three drum kits as Ms Dynamite and a Sugababe wildly bash another.
Beverley Knight is on keyboards and Daniel Bedingfield grabs a guitar. The celebratory atmosphere has returned and these scenes will be in the single's video and undoubtedly played countless times by Christmas.
AIR STUDIOS, LONDON :: 1620 GMT
The artists are starting to drift away, but The Darkness and Sugababes come into the press booth and the naughty children of the school choir cannot resist messing around.
Damon Albarn, still in pink apron, comes back and offers me a cup of tea. He asks if I take sugar.
Midge Ure, the single's executive producer, says the parts they will use from the different artists is still up in the air, and Bono is flying in from Ireland this evening to re-record the line he had in 1984.
AIR STUDIOS, LONDON :: 1745 GMT
Most artists have left and the studio is emptying. Bob Geldof and Midge Ure come back to give us a round-up of how the day has gone.
"It's been fantastic, we could ask for nothing better," Ure says.
"We've had the creme de la creme of what the UK's got to offer. The quality of the singing and the quality of the musicianship is unbelievable."
Geldof adds his tribute to the artists, adding a request for everybody to buy one - or more - copies of the single when it is released on 29 November.
"For some reason, this country throws up people who can play rock music and it's world-beating," he says.
"But the best thing of all is the attitude. What happened in this room today is properly important and memorable and will be remembered."
AIR STUDIOS, LONDON :: 1955 GMT
Bono has been delayed so I leave the studio. There is a single candlelit table set up in the centre of the hall where those still working on the song, including producer Nigel Godrich and Travis singer Fran Healy, are about to eat sushi.
A helper passes Bob Geldof two things from fans outside - a note from someone claiming to have very early Boomtown Rats tapes and an envelope simply marked "Donation".
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