Copies of Earl Spencer’s unreleased book on his sister Diana, Princess of Wales, have been stolen from a truck in the Netherlands.

The BBC understands four copies of the Dutch language edition were taken from a truck while it was en route from the Dutch publisher’s printers to their warehouse.

Dutch police said they were aware of the incident and have been approached for comment.

Swan Song: Diana, My Sister is due to be released in the UK and globally on Tuesday, and tells the story of Princess Diana’s life from the Earl’s perspective.

A source said the truck driver was taking a mandatory break in a car park and was sleeping in his cab at the time the books were stolen.

They added Dutch police are investigating. They did not say where the theft took place.

The Daily Mail, who first reported the theft, external and is serialising the book, said the theft took place earlier this week in the Dutch countryside.

The book’s publisher Penguin Michael Joseph – a division of Penguin Random House – and Dutch publisher Overamstel, who have rights to the book in the Netherlands, were approached for comment by the BBC.

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 aged 15 and 12 respectively at the time of her death.

Last month announcing the book’s publication, Earl Spencer explained ahead of the 30th anniversary of her death he had been inundated with interview requests and he wanted to write down his own thoughts and memories rather than “having the discomfort of again reading the opinions of others”.

“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.

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

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 suffering a similar fate.

In 2023, copies of Prince Harry’s memoir Spare were accidentally put on sale in Spain days ahead of the official global release.