Prince Harry was frustrated at being overshadowed by William and Kate even before he met Meghan Markle, it is claimed.
The royal "wanted out" of the pomp and ceremony, choosing instead to go into the military, a new book states.
It was meeting Meghan that emboldened him to take the step and turn his back on the family, according to authors Omid Scobie and Carolyn Durand in new book Finding Freedom.
An insider claimed Harry was left feeling like a "spare part".
They said: “Harry felt awkward as a plus one.
"They’d turn up at premieres and there was this sense that he felt a bit like a spare part.
“Long before Meghan, he wanted to change things."
Tensions between Harry and William reached breaking point in the build-up to the younger brother's wedding, when the Duke of Cambridge is said to have asked him: "Are you sure about this?"
And the Cambridges were not welcoming to Meghan when she arrived in Britain, it is claimed.
Things came to a head after his marriage to Meghan, when the couple felt "pushed out" by the Cambridges.
William and Kate were content to let negative publicity go unchallenged, while Harry and Meghan wanted to respond to every negative headline, it is claimed.
This caused a lot of stress for their advisors.
As tensions grew, the Sussexes considered the extreme measure of breaking royal protocol to contact the Queen as tensions grew in the royal family, according to the book.
An extract says Harry spoke to the Prince of Wales and the Queen about the need to change things before he left for Canada for six weeks at the end of last year.
The authors write: "He felt at once used for their popularity, hounded by the press because of the public's fascination with this new breed of royal couple, and disparaged back within the institution's walls."
While in Canada, the couple decided to step back as senior royals.
Harry attempted to set a meeting with his grandmother at the start of January, but was told she was unavailable until the end of month.
In the extract published in The Times, the authors write that as the couple flew back to the UK they "toyed with the idea of driving straight from the terminal to see the Queen".
But this was abandoned because they decided it would have "ruffled feathers" and caused them difficulty.
"At this point they felt like they had brought up the subject enough times with family members over the past year and they were fed up with not being taken seriously," a source close to the couple said.
