The mogul announced the project late last week and insisted that he would not stop trying to get more and more people involved until the very final minute.
The track – a recording of Simon and Garfunkel’s Bridge Over Troubled Water – is due for release tomorrow.
All weekend, big names have been photographed arriving at Sarm Studios to lay down their input, with the finished version expected to be released as soon as tomorrow.
Browse through the gallery below of famous faces at the location.
Kelly Jones, Carl Barat and Britain’s Got Talent winner Tokio Myers are among the newest additions to the line-up, along with The Voice UK Kids judge Pixie Lott.
It had also previously been revealed that Paloma Faith, Rita Ora, James Blunt and Gregory Porter will be on-hand.
They join the likes of Liam Payne, Lily Allen, The Who, Craig David, James Arthur, Emeli Sande, Skepta and Stormzy.
Cowell told The Sun: “Until that last note has been finished, we will not be off the phone.
“We started 24 hours ago and already we’re getting calls coming in, mainly from Brits.
“A lot of London artists, people like Skepta, Stormzy, Emeli Sande, The London Community Gospel Choir, all wanted to be involved.”
Theresa May has already confirmed that the government will waive all VAT on sales of the track, so more can go to those in need.
So far the death toll stands at 58, including those who are missing and presumed dead.
That number is widely expected to exceed 100 as investigations continue into the tragedy.