Author J.K. Rowling has added yet another short story to the Harry Potter saga.

Rowling was one of 13 authors who were invited by booksellers Waterstone’s to fill an A5 storycard to be auctioned in aid of Dyslexia Action and English Pen. She penned an 800 word hand written tale that according to newspaper reports features an early meeting between Harry's godfather Sirius Black and his real father James.

Gerry Johnson, the managing director of Waterstone's, told The Daily Telegraph: "We never dreamed that JK Rowling would donate something so precious, and we're incredibly grateful."

Unlike Rowling's most recent entry, The Tales of Beedle The Bard, copies of the new short story will be available to buy as part of a book containing all 13 storycards, on sale from August.