Liam Payne had already been confirmed for the track, while Louis Tomlinson is also thought to be roped in.
Now, it is claimed that Harry Styles and Niall Horan are scrambling to free up some time as well.
They’ll have to be quick – as bosses are hoping the cover of Bridge Over Troubled Water will be out as early as tomorrow.
According to the IBTimes, the four boys almost definitely won’t appear together due to logistics of their various locations.
However, the very prospect of them all being on the same record will be a huge coup as Cowell pushes to raise money for victims of last week’s tragic blaze.
“If all four [One Direction] came on board, it would be as solo acts as Liam has to record in the US,” a source also told The Sun.
Today, various vocalists have been arriving at a London recording studio to sing their parts.
Kelly Jones of Stereophonics, Emeli Sande, Gregory Porter, Leona Lewis, Rita Ora, Paloma Faith and Craig David are among those who have been snapped heading in.
Pixie Lott, Tulisa, Matt Terry, Louisa Johnson and Gareth Malone have also been seen.
Cowell previously 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 waived VAT on the single’s sales.