The Countess of Wessex's mother, Mary Rhys-Jones, has died in hospital at the age of 71.
Mary Rhys-Jones died on Sunday following a short illness
A Buckingham Palace statement said the charity worker had died peacefully on Sunday following a short illness.
The Countess - the Queen's daughter-in-law - was with her mother at her hospital bedside. She was "deeply saddened", the statement said.
In May, it was reported that Mrs Rhys-Jones had had a tumour removed from her colon.
The Palace said she had received treatment in hospital. Press reports said she was in the Kent and Sussex hospital, Tunbridge Wells, after an emergency operation.
The funeral will be private; arrangements for a thanksgiving service have yet to be announced.
Her daughter had been initially shocked but had played a crucial role in helping her through her ordeal, it was reported.
Prince Edward became Earl of Wessex after marrying Sophie Rhys-Jones in 1999.
In November 2003, they gave their first-born baby, Lady Louise Windsor, the middle name Mary after her grandmother.
Born Mary O'Sullivan, the Countess's mother came from a family of mainly Irish shopkeepers and farmers.
As a young woman she once danced with Prince Philip at a society ball.
She lived with her husband, Christopher, a retired salesman for a tyre maker, in a four-bedroom 17th-century farmhouse in the west Kent village of Brenchley.