The 33-year-old beauty has been with the rugby player since meeting him in a nightclub in November 2010.
But they broke up last week after Evans grew jealous of Brook's friendship with her former boyfriend Danny Cipriani, reported Sun online.
"She's never stopped loving Danny. Even though there was nothing going on while she and Thom were together, Thom hated the fact she was seeing him and Kelly refused to end their meetings," a source said.
"After one row too many the pair decided to end things. She's been leaning on Danny since the break-up and, while she's devastated, she knows it's for the best," it added.
Brook, who has also dated actors Jason Statham and Billy Zane, will continue to live in their London home, while Evans, 27, has already moved to Los Angeles where he is hoping to start an acting career.