A spokesman for Simon said in a statement: "Syco Music will no longer work with Modest Management and therefore any artists signed to that management company.
"We do, of course, wish all artists affected by this decision every future success.
"Consequently, LM5, the forthcoming Little Mix album A&R’d and released on Syco Music, is to be serviced by RCA at the request of Syco and Simon Cowell."
Reports suggest that tensions began rising between the girls and Syco when they asked for writing credit on their new single, "Woman Like Me" alongside Steve Mac, Jess Glynne and Ed Sheeran.
They were not looking for publishing money for their work.
Allegedly, when they took their issue to Simon in a "strongly-worded email," things escalated and Modest became involved. Syco then terminated their working relationship with Modest.