Copies of Earl Spencer's forthcoming memoir about his sister Princess Diana have been stolen from a lorry days before its exclusive serialisation in the Daily Mail, in what publishing sources describe as an unprecedented breach of security. The book, Swan Song: Diana, My Sister, contains explosive claims about Diana's unhappy marriage to the then Prince of Wales and reveals how the King reacted to her death, and its theft has prompted a Dutch police investigation amid speculation ranging from opportunistic thieves to a royal sympathiser deliberately targeting the manuscript.

The lorry, carrying thousands of copies from a printing plant in central Europe to a warehouse in the Netherlands, was broken into overnight while parked in a lay-by, with only four black-plastic-wrapped copies taken despite the driver being asleep nearby. Penguin Michael Joseph, the UK publisher, was not informed until 3.30pm the following day, and the book remains on track for worldwide publication next Tuesday, translated into 12 languages. Earl Spencer, 62, has said the memoir will correct "untruths" about his sister ahead of the 30th anniversary of her death, echoing a previous incident in 2003 when 7,000 copies of a Harry Potter novel were stolen before release.