LONDON, June 3: The late Princess Diana's mother, whose own unhappy youthful marriage at the top of Britain's aristocracy mirrored that of her daughter, died on Thursday after a long illness, her family said.

Frances Shand Kydd was 68. "Earl Spencer's mother passed away peacefully this morning after suffering from a long illness. Now this is a private time for the family to grieve," said a spokesman for Diana's brother Earl Charles Spencer.

Prince William, second in line to the throne, said in a statement that he and brother Harry were "very upset by the death of our grandmother and we will miss her a lot".

Queen Elizabeth sent a private message of condolence to the Spencer family, and Diana's former husband Prince Charles said he was very saddened by the death. Shand Kydd, who lived alone on an island in Scotland, was estranged from Diana at the time of her death in a Paris car crash in 1997, and had not spoken to her daughter for months.

But she told biographers that she viewed the rift as temporary. "Of course we argued. Who wants a wishy-washy mum," Shand Kydd told authors Max Riddington and Gavan Naden. Britain's press made much of parallels between the lives of mother and daughter - both of whom were thrust into the spotlight in glamorous but ultimately unhappy marriages with men far older and higher up in the aristocratic hierarchy.

The daughter of a baron, Shand Kydd was still a teenager when she married the future Earl John Spencer in 1954. She caused a scandal 15 years later when she ran off with wallpaper millionaire Peter Shand Kydd, and later lost custody of her children in an acrimonious battle.

But she was among the world leaders and dignitaries assembled in London's St Paul's Cathedral when her daughter, barely 20, married heir to the throne Prince Charles in 1981. -Reuters