Charles Spencer, 9th Earl Spencer, discussed his new book about his sister Princess Diana, and the controversy surrounding it, on Good Morning America.

Spencer’s book, Swan Song: Diana, My Sister is scheduled for publication Tuesday by Penguin Press. Earlier this month, the Daily Mail printed excerpts from the book, which includes Spencer’s claim that King Charles III—then the Prince of Wales and Diana’s ex-husband—had called him after Diana’s death, sounding “giddily elated—like a lottery winner.” Spencer also reports that Charles told him, “Rest assured, we’ll forget her soon enough!”

Buckingham Palace has responded to the claim with a public statement: “His Majesty is mindful that the pain of fraternal grief can cloud reason, affect judgement and color memory in ways others do not recognize.”

“It’s not tittle-tattle,” Spencer told ABC News’ Maggie Rulli. “I’m not trying to damage the monarchy or whatever I’ve been accused of doing. But I wanted to do this properly, once and for all, for Diana, and remind those who knew her when she was alive why they connected with her.”

Rulli noted that Buckingham Palace usually does not respond to books about the royal family.

“Well, it’s not every day you’re gaslit by a king, but I was really upset by that, because I thought it was not only condescending, but not straight,” Spencer said. “What I said was straight and true, and what do they come back with? That? It doesn’t say he didn’t say it, either. That’s what I found interesting. But I put that in the book because it’s important. It’s a fundamental part of the truth of my book.”

Michael Schaub is a contributing writer.