Earl Spencer, brother of the late Princess Diana, has detailed his observations of a perceived lack of adequate care for his nephew, Prince Harry, in the immediate aftermath of Diana's death.

In excerpts from his forthcoming book, "Swan Song: Diana, My Sister," Spencer recounts walking behind his sister's coffin as one of the most harrowing experiences of his life, still causing him nightmares. He described the profound emotional impact of seeing the then 15-year-old Prince William and 12-year-old Prince Harry walking alongside him, Prince Philip, and Prince Charles.

Spencer focused on Harry's appearance, writing, "I could not get over how tiny, and slight, and bereft, Harry looked. Too small to be doing this, I thought, and wondered how on earth this heartbroken child could bear the ordeal of that moment, or how he could possibly be expected to deal with the agony that the day ahead would insist upon."

While he noted William appeared "more self-possessed," Spencer acknowledged his older nephew was also enduring immense suffering. "He was the image of his mother then: tall and handsome and dignified. Although I cared deeply about what he must be going through, his greater composure and maturity made him my secondary concern that morning," Spencer wrote.

During a moment when the procession momentarily moved out of public view, Spencer recalled offering a word of encouragement to Harry, telling him, "You’re doing bloody brilliantly!"

Following the funeral service at Westminster Abbey, the somber train journey to Diana's family estate, Althorp, left Spencer with a lasting concern. "My chief memory of the journey is of being concerned at the lack of obvious care for Harry, as he decompressed from the ghastliness of the morning," he stated.

Spencer expressed his view that Harry was denied the comfort he needed. "He needed Diana’s hugs, of course, and they would forever be denied him," he wrote. "But instead of calm and nurture, he was allowed to spin out of control. I have often looked back, reckoning that a more caring hand at such points as this would have helped him enormously in the future. Instead, he seemed so very alone in his despair and torment."

Buckingham Palace, in response to earlier claims made by Spencer, issued a rare statement: "While we do not comment on books as a matter of principle, His Majesty is mindful that the pain of fraternal grief can cloud reason, affect judgment and color memory in ways others do not recognize, even many years after such a loss." Representatives for the palace and Prince Harry did not offer further comment when reached by HuffPost.

"Swan Song" is scheduled for release on September 22.