Earl Spencer is to publish a new book telling the life story of his sister, Diana, Princess of Wales, from his perspective.

Swan Song: Diana, My Sister is due to be released in the UK next month, nearly 30 years after she died.

"I want to speak on Diana's behalf and do so in an openly positive way," Earl Spencer said of his reasons for writing the book.

He added that he hoped it would enable people to understand 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 far better place, despite exiting the stage while still so young."

Princess Diana, who died aged 36 following a car crash in Paris in 1997, would have turned 65 on 1 July this year.

Her sons, Prince William and Prince Harry, were 15 and 12 years old, respectively, at the time of her death.

In a statement released on Tuesday to mark the new book, Earl Spencer said he had been "inundated yet again" with requests for interviews about his sister, with the 30th anniversary of her death approaching.

"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," he said.

"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," he added, referring to the eulogy he delivered in Westminster Abbey in September 1997.

He said he hoped the book would be of interest to those who "still cherish" Diana's memory from 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," he continued.

In his eulogy, Earl Spencer described Princess Diana as "the very essence of compassion, of duty, of style, of beauty", adding: "All over the world she was a symbol of selfless humanity... a standard bearer for the rights of the truly downtrodden."

He also used the speech to criticise the media, calling her the "most hunted person of the modern age" and pledged to protect Prince William and Prince Harry and prevent them from suffering a similar fate.

The book is expected to be released in the UK on 22 September and will be published by Penguin Michael Joseph, a division of Penguin Random House.

Daniel Bunyard, publishing director at Penguin Random House UK, said: "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."