Charles Spencer will publish a book about the life of his sister, Diana, Princess of Wales, which contains “revelations that will draw considerable attention”.

Swan Song: Diana, My Sister will mark the first time that Earl Spencer, 62, has spoken at length about his sibling, three decades after her death.

Speaking about the book, he said: “With the 30th anniversary of Diana’s tragic death on the skyline, I’ve been inundated yet again with requests for interviews about my sister.

“This has convinced me to write down my own thoughts and memories, once and for all, rather than having the discomfort of again reading the opinions of others, many of which are based on untruths that have become accepted over time.

“The aim of this book is to tell the truth about Diana from her brother’s perspective, just as I attempted to do when speaking at her funeral.

“As with my eulogy, I want to speak on Diana’s behalf and do so in an openly positive way.

“I hope this book will be of interest to the many who still cherish the memory of Diana when she was alive, and I would be delighted if those too young to remember her in her heyday feel – after reading this – that they ‘get’ who she really was: a one-off dynamic character, full of love, charisma and self-doubt, who gave life a hell of a go, and left the world a better place, despite exiting the stage while still so young.”

During Diana’s funeral in 1997, Earl Spencer delivered a passionate eulogy in which he pointedly referred to his sister’s “blood family”.

Daniel Bunyard, publishing director at Penguin Random House UK, added: “This is a book of historic importance, telling as it does the story of one of the 20th century’s most iconic and culturally significant figures, but it is also utterly unique in being written by Earl Spencer, from a vantage point of such intimacy.

“Charles Spencer is both a loving brother and an acclaimed memoirist as well as a celebrated historian, and all these facets are brought together in Swan Song.

“It is extraordinarily moving, beautifully written and remarkably candid, in no small part due to Charles’ rare ability to access great emotional depth without sentimentality, honest observation without rancour, and warmth without gloss.

“While these pages contain many revelations that will draw considerable attention, readers will find, for the first time, but also for all time, a fitting tribute to the beloved woman who was Princess Diana.”

Born into the aristocratic Spencer family in 1961, Diana married the King, then Prince of Wales, in 1981 and had sons William and Harry. The couple divorced in 1996.

She died in August 1997 in a car crash in Paris at the age of 36. Her partner, Dodi Fayed, and their chauffeur Henri Paul also died in the crash.

Her death led to widespread public mourning and her funeral was watched by more than two billion people worldwide.

Swan Song: Diana, My Sister will be published on September 22.