The Take That frontman, 45, is said to be launching a new talent show to rival Simon's The X Factor.
And Simon, 56, is allegedly furious - awks!
According to The Sun, Gary's new Saturday night series will replace The Voice after it moves to ITV.
The rumoured multi-million pound project, Gary's own format, will apparently compete directly with The X Factor - the show Gary judged from 2011 to 2013 until he left to concentrate on his music career.
A TV source told the newspaper: "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 alongside Mark Owen and Howard Donald.
During last year's series of The X Factor, the show was plagued by reports of failing ratings, while co-hosts Caroline Flack and Olly Murs came under fire.
This week, Nick Grimshaw stepped down from his role as judge after just one year. Rumours quickly followed that Caroline is also set to quit.
It has also been reported that Rita Ora may desert The X Factor and return to The Voice.
Meanwhile, Take That are reportedly planning to reunite with Robbie Williams and Jason Orange in time for their 25th anniversary.
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