For the first time, the mogul decided to add extra artists to the contestants singing the song — a version of Rose Royce classic Wishing On A Star.
The 16 acts who made the live shows recorded their parts last month. But the decision was later made to add the chart-topping former X Factor stars to boost sales.
An insider said: „Simon and the other SyCo execs want huge sales this year and weren’t sure the contestants would get that on their own.
„Now they’ll be using One Direction and JLS’s combined fanbase to sell the song. It’s a no-brainer and is bound to be a huge seller now.”
Money raised through sales of the track — due out on November 20 — will be handed to kids’ charity Together For Short Lives.
Announcing the single last month Cowell said he had picked the charity because of its „amazing work for kids„. The insider added: „The charity was personally chosen by Simon himself so he wants the song to shift huge numbers.”
Yesterday a spokesman said: „As Simon has a personal involvement in this year’s charity he has asked two of the country’s biggest acts to make this year’s X Factor charity single the biggest of the year.
„He asked One Direction and JLS to sing on the track to give the song something extra special — and encourage all their fans to buy the single and raise lots of money for the charity.„
Every previous X Factor charity single has hit No1 and raised substantial sums for good causes.
Hero reached the top spot in 2008, followed by a version of Michael Jackson’s You Are Not Alone a year later. A cover of David Bowie’s Heroes did the same last year.