Countess Mountbatten of Burma has died at the age of 93.
Born Patricia Mountbatten, the Countess was the Duke of Edinburgh's first cousin.
A Buckingham Palace spokeswoman said: "The Queen and the Duke are aware and have privately passed on their condolences."
The Countess's father Earl Mountbatten, her 14-year-old son Nicholas Knatchbull and her mother-in-law the Dowager Lady Brabourne were all murdered by the IRA in 1979 when their boat was blown up off the coast of Sligo.
The Countess, then known as Lady Brabourne, suffered serious injuries, but survived the blast, as did her husband Lord Brabourne and Nicholas's twin brother Timothy.
A local boat boy, 15-year-old Paul Maxwell, also died.