Spencer has maintained his memoir reflects his genuine recollections rather than an attempt to attack the monarchy.
Speaking about Diana, he said: “My memories of Diana tend to be from our happy childhood days.”
The controversy comes almost 30 years after Diana’s death and has reopened debate over the royal family’s handling of the extraordinary days between the Paris crash and her funeral.
Spencer has also addressed the fractured relationship between his nephews, William, 44, and Harry, 42, insisting he does not intend to choose between them.
He said: “I’m on my deceased sister’s sons’ side, both of them. And if either of them wanted anything, then I’d be equally there for them.” The memoir is due to be published on September 22, and Spencer has said its purpose is to record his memories of his sister.
Buckingham Palace’s intervention, meanwhile, leaves the central allegation disputed – Spencer maintains his recollection is accurate, while the Palace says grief can affect judgment and memory.