Princess Diana will be remembered through the eyes of her younger brother in a forthcoming book that promises a deeply personal account of her life, character and legacy. Earl Spencer is preparing to publish ‘Swan Song: Diana, My Sister’, a memoir that he says will set down his own memories of Diana and challenge what he believes are long-standing misconceptions about her.

The book is scheduled to be published in the UK on 22 September, nearly three decades after Diana died in Paris in 1997.

Explaining why he decided to write the book, Earl Spencer said he wanted to speak positively on his sister’s behalf and help readers understand the person behind the public image.

He described Diana as an extraordinary and complicated figure, full of affection, charisma and uncertainty, someone who embraced life intensely despite dying at such a young age.

Diana was 36 when she died following a car crash in Paris. She would have turned 65 on 1 July this year. Her sons, Prince William and Prince Harry, were 15 and 12 at the time of her death.

Earl Spencer said the approaching 30th anniversary of Diana’s death had once again brought a flood of requests for interviews about her.

Rather than repeatedly responding to other people’s versions of her life, he decided to record his own perspective in one place.

He said he had become increasingly uncomfortable seeing claims about Diana repeated over the years, particularly when he believed some of them were based on inaccuracies that had gradually become accepted as fact.

For Spencer, the book is intended as a permanent account of what he remembers and believes to be true about his sister.

Spencer has linked the purpose of ‘Swan Song’ to the speech he delivered at Diana’s funeral in Westminster Abbey in September 1997.

In that eulogy, he described Diana as someone defined by compassion, duty, beauty and humanity. He also criticised the intense media attention that had followed her throughout her life, famously portraying her as one of the most relentlessly pursued public figures of the modern era.

The speech also included his promise to protect William and Harry from experiencing the same pressures.

Earl Spencer said he hopes the book will resonate with readers who still remember Diana vividly, but also with younger generations who know her mainly through documentaries, photographs and second-hand accounts.

His ambition is for readers who never witnessed Diana at the height of her public life to come away with a clearer understanding of who she was beyond the mythology.

That goal places ‘Swan Song’ somewhere between memoir, family remembrance and an attempt to reclaim a public narrative that has been shaped by decades of commentary.

Penguin Random House has suggested the book will contain material likely to attract considerable attention. Daniel Bunyard, publishing director at Penguin Random House UK, said the memoir would include revelations while also serving as a lasting tribute to Diana.

Nearly 30 years after her death, public fascination with Princess Diana remains remarkably strong. ‘Swan Song: Diana, My Sister’ now adds another voice to that continuing story, but this time from someone who knew her not as a royal icon, but simply as his sister.