Martin Pengelly, a journalist for the Guardian's US website who wrote its story, said he did not approach Prince William's communication team.
The reporter said that his article is "a report on Harry's book, which he's written, it's Harry's account".
Mr Pengelly told BBC Radio 5 Live: "We carefully, obviously in reporting it, didn't call it a fight because Harry says he didn't fight back."
Prince Harry writes that his brother urged him to hit back and he refused to do so, according to the Guardian, but Prince William later looked "regretful, and apologised".
Photographs suggest Prince Harry regularly wore a dark necklace at events such as the Invictus Games, and on foreign tours with Meghan, as recently as September 2019.
While publishers at Penguin Random House are yet to confirm whether the leaked excerpts from the book are genuine, Prince Harry has recently spoken of his troubled relationship with his brother.
In the couple's Harry and Meghan Netflix documentary, Prince Harry describes a meeting he attended with his brother, and father, the now King.
Prince Harry described the conference in early 2020, which was also attended by the late Queen, as "terrifying".
"It was terrifying to have my brother scream and shout at me and my father say things that just simply weren't true, and my grandmother quietly sit there and sort of take it all in," he said.
The Guardian says Prince Harry details a meeting with Charles, then Prince of Wales, and Prince William after the funeral of his grandfather, Prince Phillip, in April 2021.
According to the paper, Prince Harry writes his father stood between him and Prince William, and said "please, boys, don't make my final years a misery".
In a trailer for a sit-down interview, which will be broadcast ahead of the book release on 8 January, the prince said: "I would like to get my father back, I would like to have my brother back".
However, Prince Harry told ITV's Tom Bradby "they've shown absolutely no willingness to reconcile," although it was not clear who he was referring to.
Buckingham Palace declined to comment on this.
Spare, ghostwritten by memoirist JR Moehringer and part of a multi-million dollar book deal, was previously believed to be subject to the upmost secrecy with few details known about its content.
"For Harry, this is his story at last," Penguin Random House said in a publicity statement back in October.
"With its raw, unflinching honesty, Spare is a landmark publication full of insight, revelation, self-examination, and hard-won wisdom about the eternal power of love over grief."
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