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The 62-year-old earl shared photographs of Althorp House with the flag lowered to half-mast

Earl Charles Spencer paid a poignant tribute to his late sister Princess Diana on Monday, sharing photographs of Althorp House on Instagram with the flag lowered to half-mast.

The 62-year-old earl accompanied the images with a brief but deeply personal message: "31 August. My least favourite day."

The post, which quickly attracted thousands of reactions but did not allow comments, marked 29 years since Diana died in a Paris car crash in the early hours of August 31, 1997.

Althorp House in Northamptonshire holds profound significance for the Spencer family - it was Diana's childhood home and remains her final resting place.

The former Princess of Wales is buried on a small island at the centre of an ornamental lake known as the Round Oval within the estate's grounds.

Diana was 36 when the car she was travelling in crashed in the Pont de l'Alma tunnel in Paris, as it attempted to outrun pursuing paparazzi photographers on motorbikes.

Earl Spencer has previously spoken of the solace he takes from knowing that both Prince William and Prince Harry can visit their mother's burial site in private whenever they wish.

"I think it's very important for them to be there with her," he told People. "It is, luckily, very tranquil here, and they can come and go as they wish whenever they want."

The anniversary falls just days after Prince Harry relocated back to Britain with his family. His wife, Professor Cat Spencer, publicly liked the post.

Earl Spencer's tribute comes shortly after he revealed plans for a new book dedicated to his sister's life.

Titled Swan Song: Diana, My Sister, the work is set for publication on September 22, positioning it ahead of the 30th anniversary of her death next year.

Published by Michael Joseph, an imprint of Penguin Random House, the book will draw on Earl Spencer's unique vantage point as both a historian and Diana's only brother.

"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," Earl Spencer said in a statement.

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

He added that he had been "inundated yet again with requests for interviews about my sister" as the 30th anniversary approaches.

Royal commentator Jennie Bond has suggested that a joint appearance by William and Harry to honour their mother remains unlikely this year, though she has raised the possibility of the brothers coming together for commemorations marking the 30th anniversary in 2027.

For now, Ms Bond expects both princes to remember Diana privately and in their own ways.

Years of well-documented tension between the brothers have continued to overshadow significant family moments. Some within royal circles, however, believe Harry and Meghan's recent return to the UK offers the "best opportunity" for reconciliation in some time.

A source close to the royal household told People: "It would never happen with him in America. This gives them the opportunity to try and mend that bridge."

The source added that King Charles "will want to see his sons together over time."