Simon and Gary's feud started in 2011
Simon asked Gary to be a judge on The X Factor
"Gary’s musical pedigree is second to none – and we have known each other a long time now – so I am delighted he is picking up the baton for me for the first series of Walk The Line. I know he will do a fantastic job."
But the pair of them must have put their differences aside because they seemed to hit a rocky patch when they started working together in 2011.
In 2011, Simon and Cheryl had jetted over to the US to work on the American version of The X Factor which meant Simon needed a replacement judge so he turned to Take That singer Gary.
He then worked alongside the other judges for that year, Tulisa Contostavlos, Nicole Scherzinger and Louis Walsh.
However it seems the friendship didn't last long as when Gary was awarded an OBE in 2012, it was alleged that Simon became jealous of his achievement.
When Gary left The X Factor he said he couldn't fit it into his schedule
Simon's unauthorised biographer Tom Bower wrote: "The announcement that Barlow would receive an OBE for arranging the event added to his irritation. 'He chased that madly to get three initials after his name'.
"The ambitions of Barlow, he believed, were comical."
He said that after a 'disastrous' attempt for Gary to be a solo star, Simon was concerned about him trying to build a rival entertainment empire.
Tom added: "He enjoyed snubbing the man who foolishly asserted that he was the top judge, not Louis Walsh, by texting his criticism about Barlow to others."
In 2012, Simon returned to the talent show to judge, replacing Gary again but he said he was leaving to focus on his music career instead.
At the time he said there wasn't any time for him to fit The X Factor into his schedule.
When Gary judged the show, viewing figures went down to 2006 levels which Simon reportedly blamed on him.
Simon took a break from judging the UK's X Factor
It seems the feud was going on for a few years as Simon threatened to sue Gary in 2017.
This was due to Gary planning a talent show to find people to be in a Take That musical.
He was a judge on the show as well as former X-Factor judge Dannii Minogue and Spandau Ballet singer Martin Kemp.
But Simon was convinced the show was too close to The X Factor but Gary denied the similarities.
However, Simon dropped the case when lawyers told him that it would be difficult to win the case.
A source told The Sunday People : "Simon’s team was keeping a close eye on Gary’s show.
"They want to make sure that it’s completely different to his own projects. Simon has since been advised to abandon his case against Let It Shine.
Gary will be hosting Walk The Line
"He is very serious when it comes to protecting the intellectual property of his shows. He trailblazed in the TV talent show sector and has to protect his brand. But it’s very hard to win these kind of cases and his legal team has gone back to him to say any action would most likely be fruitless."
Gary will be joined by Dawn French, Alesha Dixon and Craig David as judges on Walk The Line.
The new musical talent show will see contestants compete to win a life changing amount of money.
Maya Jama will be hosting the show which starts this Sunday 12th December at 8pm on ITV and ITV Hub.