Mrs Susan Barrantes, the mother of the Duchess of York, Sarah Ferguson, was decapitated in a head-on collision on an Argentine provincial highway on Saturday night, police said yesterday.
Mrs Barrantes' Land-Rover and a light truck collided in flat countryside, 520km west of Buenos Aires.
Her nephew, Mr Rafael Barrantes (25), who was in the Land-Rover, and the other driver, Mr Jose Maria Rodriguez (57), both survived.
Mrs Barrantes (61) "was decapitated" and died instantly, when her green Land-Rover and the other vehicle collided, the police said. She was driving back to her country home, about 80km from the scene of the crash, after lunching with friends in a nearby town.
"We can presume the crash was head-on due to blinding by headlights or one of the drivers becoming distracted and crossing into the oncoming lane," a police spokesman said. He added that it was a windy night with an unusually high volume of traffic.
Mrs Barrantes' death comes a year after a high-speed car crash killed Princess Diana and her companion, Dodi Fayed, in Paris in a case that is still being investigated. Like Princess Diana, Mrs Barrantes was not wearing a seat belt at the time of the crash, a Ferguson spokeswoman, Ms Kate Waddington, said in London.
Mrs Barrantes was married for 18 years to Maj Ronald Ferguson, Prince Charles's polo manager, before she left him and their two daughters and married a professional Argentine polo player, Hector Barrantes, who died of cancer in 1990.
"It is an absolute tragedy and I'm extremely sorry. I'm very upset by it," Maj Ferguson said of his former wife's death.
Mrs Barrantes' daughter, Sarah Ferguson, is the former wife of Prince Andrew, the second son of Queen Elizabeth.
Queen Elizabeth, who is on a visit to Malaysia, was informed of Mrs Barrantes's death. "They were very shocked and very saddened at the news," a spokesman said.
On receiving word of her mother's death, the duchess flew from a holiday in Italy to London and was scheduled to fly to Argentina last night with her uncle for the funeral.