The 9th Earl Spencer, Sept. 23 -- Charles Spencer described his Tuesday morning with a precision that left little room for diplomatic interpretation. He had woken, he said, to find he had been gaslit by the king.

Diana, My Sister, which went on sale in Britain on Tuesday. The palace issued what courtiers acknowledged was an unusually direct public response for an institution that has historically declined to dignify royal books with comment. The statement questioned his memory. It did not deny his account.

Spencer's book, published by Michael Joseph, an imprint of Penguin Random House, makes two claims about King Charles III's conduct in the hours and days after Diana's fatal car crash in Paris on August 31, 1997. In the most incendiary...