According to The Sun, the couple went their separate ways after she decided she was too young to marry.
The Strictly Come Dancing star, 28, sparked rumours of a split when she attended Sunday's BAFTA Television Awards without her engagement ring.
A source said: "After much soul-searching, Kelly decided that Billy is not the man she wants to marry and has called off the engagement.
"The split is completely amicable and the decision was made after long conversations here and in the US. They remain good friends."
The couple, who dated for four years, are believed to have ended their romance after Brook repeatedly refused to set a date for their wedding.
The insider said: "Billy was keen but she kept delaying plans for the big day. He started asking questions - and they eventually decided to go their separate ways."
Zane, 42, has now left their Kent farmhouse and is moving back to LA for good.