Letters from the late Diana, Princess of Wales, and from Prince Philip are being published in Paul Burrell's new book, causing Buckingham Palace lawyers to take notice. But who owns a letter once it's been sent?
Former royal butler Paul Burrell is in the headlines again, this time for "going public" with a new set of revelations concerning the break-up of Prince Charles' marriage to the late Diana, Princess of Wales.
As well as publishing letters written from the princess to himself, Burrell has included in his book letters written by Prince Philip to his former daughter-in-law as the marriage disintegrated in the early 1990s.
The Palace has been sent a copy of Mr Burrell's new book, which goes on sale next week, so it can consider legal implications. The Daily Mirror has been serialising excerpts from it this week.
But who owns a letter once it has been sent? Can the Palace lawyers do anything to stop the book being published?
An important distinction is that between a letter itself (i.e. a piece of paper) and the words written on that paper.
The ownership of the letter, as opposed to the words, is with the person it was sent to. The princess would have owned the letters sent to her by Prince Philip. After she died, they would normally have passed to her estate - Mr Burrell claims he was given them by Diana.
The words in the letters, however, remain the "property" of Prince Philip. He retains copyright in the same way that while you own the books on your shelf, the authors still own the words.
The copyright in the words written by Diana would, in principle, be owned by her estate, and would continue to do so until 70 years after her death.
As far as Prince Philip's words are concerned the publishers of Mr Burrell's book will undoubtedly be relying on the defence of "criticism, review and new reporting".
This allows use of relevant sections of copyright material to be reprinted for comment on them. Whether or not the Mirror and Mr Burrell's publishers have stepped over that mark would be for a court to consider.
"There's not a lot of precedent in this area," says copyright lawyer Elizabeth Ward. "The test would be: would a fair-minded man consider a particular piece had been added for titillation, for example, rather than criticism and review."
If a court found in favour of the Palace, it could then issue an injunction on the book, stopping publication. But it can't turn back the clock on the extracts published in the Mirror, and could instead force the publisher to pay a royalty.
The second option open to Palace lawyers is that of confidentiality. If they can prove these letters were purely confidential, they could again force an injunction.
On the face of it, this would be a strong challenge. Personal letters are by their nature confidential. The fact Philip allegedly signed them "With fondest love, Pa" adds to sense of the intimacy.
But "this is not an absolute" says Ms Ward. The acrimonious divorce of the Duke and Duchess of Argyll in the 1960s, established that letters between spouses were a special case, and strict confidence was implied.
But in the Diana case, the letters are from her father-in-law.
The only sure-fire way of ensuring secrecy otherwise is with the words "private and confidential", but Prince Philip is not thought to have included this caveat in his letters.
At best for the Palace, it is a grey area. After the collapse last year of theft charges against Mr Burrell, would the Palace would want to take him on in open court again?