To this day, royal experts continue to speculate over the reason for their brief breakup, with some suggesting Kate's parents, Michael and Carole Middleton as a factor.
In the royal documentary,William & Kate: The Journey, royal editor Duncan Larcombe theorised that Kate's family background played a significant role in the background and that William's friends used to mock them.
According toExpress, Buckingham palace has denied these claims, but Larcombe believes that "One or two people in William’s inner circle may have had that view."
In particular, friends of the Duke of Cambridge would ridicule Kate's mother, Carole (who once worked as an air hostess).
"They used to say ‘doors to manual’ whenever they saw Kate Middleton," said Larcombe.
WATCH: Prince William says he regretted break up with Kate Middleton
In his book, Game of Crowns, Christopher Anderson explores the lives of three of the most powerful Windsor women (Queen Elizabeth II, Camilla and Kate).
Royal insiders told Anderson that Camilla was largely involved in Kate and William's decision to split.
"People started telling me that Camilla was behind it," he said.
According to Anderson, the Duchess of Cornwall was "a bit of a snob."
"She had always thought of herself as the heiress to Alice Keppel, her great-grandmother, who was the mistress of Edward VII," he said.
"She was very proud of that connection, she boasted about that as a child and as an adult and that’s what she intended to be; part of the Royal circle in the role of mistress to the future king, and then the king."
As a result, the second wife of Prince Charles allegedly didn't see William's future wife as worthy of joining the Royal family.
Prince William's friends would allegedly ridicule Kate Middleton's mother, Carole Middleton.
"Kate is the first working-class woman to be accepted into the Royal Family," said Anderson.
"So for all those reasons Camilla never really felt that Kate Middleton as an individual and the Middleton family as a whole were going to be worthy of entering into the Royal Family."
According to Anderson's royal sources, Camilla whispered in the Prince of Wales' ear and instigated William and Kate's first break up.
"It has been since confirmed publicly that Charles did suggest to William that he either make a commitment to Kate or basically set her free, as it were," said Anderson, referring to the moment Prince Charles had a serious conversation with his eldest son about his relationship with Kate.
"Now, his motives for doing that may have been pure but Camilla’s…not so much. She was the instigator of this."
Camilla Parker Bowles allegedly didn't see Kate Middleton as worthy of joining the royal family.
However, her attempt to drive a wedge between William and Kate failed, as the couple reconciled soon after, and their engagement was announced in 2010.
Their royal wedding was held at Westminster Abbey in London the following year, and they are now happy parents to Prince George, Princess Charlotte and Prince Louis.