The Take That frontman, 45, is reportedly launching a new talent show to rival Simon's The X Factor. And Simon, 56, is allegedly ‘furious’.
The Take That superstar’s series is lined up by the BBC to replace The Voice, which is moving to ITV after its current Saturday night run.
His as yet unnamed multi-million pound programme will compete directly with The X Factor, on which he was a judge from 2011 to 2013.
Talking to The Sun, a source said: “This is Gary’s own format that will be made by the BBC entertainment team.
“If it is a success he could make hundreds of millions of pounds around the world like Cowell has with X Factor and Britain’s Got Talent.
“Cowell is going to be furious because he thinks it was X Factor that gave Gary the experience working on a show like this. Now he’s gone and set up a direct rival.
“He’s always been paranoid about Gary trying to set up a huge entertainment company incorporating music and TV, which seems to be happening.”
Sources close to Gary said he was "very excited" about the project, but still planned to remain a fully-committed member of Take That.
The band are reportedly planning to reunite with Robbie Williams and Jason Orange in time for their 25th anniversary.