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CloseEarl Spencer, brother of Diana, Princess of Wales, said he misses his sister every day during an interview with the BBC.
"That's the thing with sibling loss, you never get over it," he told Laura Kuenssberg.
His book, Swan Song: Diana, My Sister, is due to be published on Tuesday and is expected to be highly critical of the Royal Family.
He said one of the reasons the book is called Swan Song is because it is his "sign off" as Diana's brother and "final take on the situation".
"This is my swan song too. I'm not here to bang on about Diana, this is my final word, celebration of her," he added.
The full interview, Princess Diana: My Sister's Story, is available on BBC iPlayer and will be broadcast on BBC One at 18:30 BST on Sunday.
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