“Swan Song: Diana, My Sister” will be published in Britain and other countries on September 22 and translated into 12 languages.

Charles Spencer, the brother of the late Princess Diana, revealed Tuesday he has penned a book “to tell the truth” about his sister and her life as the 30th anniversary of her death looms.

Spencer “tells the astonishing story of his life with Diana and the tragic week of her death,” Penguin Random House said in a statement, adding it marked “the first time that Earl Spencer has spoken openly and at length about his sister”.

“Swan Song: Diana, My Sister” will be published in Britain and other countries on September 22 and translated into 12 languages.

“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,” Spencer said.

“This has convinced me to write down my own thoughts and memories, once and for all,” he said, adding many opinions were “based on untruths that have become accepted over time”.

“I want to speak on Diana’s behalf and do so in an openly positive way,” added Spencer.

After Diana was killed in a Paris car crash in August 1997, Spencer gave an emotional eulogy at her funeral in Westminster Abbey, London, vowing to always look out for her two sons, Prince William and Prince Harry.

Thirty years on, Diana remains an iconic figure, but Harry and William, now heir to the UK throne, are no longer speaking amid a major rift in the royal family.

And the husband Diana divorced is now the UK’s King Charles III and married to his former mistress, Queen Camilla.

“The aim of this book is to tell the truth about Diana from her brother’s perspective,” Spencer said.

Spencer, 62, has remained close to the two princes, and Harry, his wife Meghan and their two young children reportedly visited the earl’s ancestral home at Althorp, central England, in early July where they were said to have visited Diana’s grave.

Spencer added he hoped his book would interest those who “cherish” her memory and appeal to “those too young to remember her in her heyday”.

He wanted to leave readers with the impression of 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”.

Publisher Daniel Bunyard said it was “a book of historic importance” recounting the “story of one of the twentieth century’s most iconic and culturally significant figures”.