Prince William, Kate Middleton break-up: Will dumped Kate over the phone while she was at work

Details of Prince William and Kate Middleton's early relationship will form part of a new book on the royals. Photo / AP

Details of the Duke and Duchess of Cambridge's brief split in 2007, four years before they tied the knot, have been revealed in a book.

Royal expert and author Robert Lacey has revealed that Prince William broke up with girlfriend Kate in a phone call, while she was at work.

In the new book Battle of the Brothers, Lacey says William, who was 25 at the time, was feeling "uncertain" about his future with Kate.

In this Handout Image provided by Clarence House, Kate Middleton and Prince William on the day of their graduation ceremony at St Andrew's University in St Andrew's on June 23, 2005 in Scotland. Photo / Getty Images

At the time, Kate worked at fashion store Jigsaw, as an accessories buyer. William reportedly called Kate while she was at work.

"Kate excused herself from a meeting at Jigsaw, the fashion store with which she'd recently started working, to take a call from William in a room out of earshot of the other buyers," Lacey wrote, according to a book excerpt published in the Daily Mail.

"She shut the door for more than an hour. When she came out, she was single."

The split only lasted a couple of months.

The couple later addressed the split in their engagement interview in 2010.

"We were sort of both finding ourselves as such and being different characters and stuff, it was very much trying to find our own way and we were growing up," Prince William said at the time.

"At the time, I wasn't very happy about it but actually it made me a stronger person," Kate shared.

"You find out things about yourself that maybe you hadn't realised, or I think you can get quite consumed by a relationship when you are younger and I really valued that time for me as well although I didn't think it at the time, looking back on it."