Princess Diana feared she looked like an elephant when she danced, and was relieved when her elder sister did not marry Prince Charles, letters set to be auctioned later this month show.

In the private letters to her nanny, which will be sold on September 30 by auctioneers Reeman Dansie, she also wrote of how, during her pregnancy with Prince William, she longed "for the day when I can eventually sit on the loo (rather than) looking over it!"

Born into an aristocratic family, Diana's engagement to Prince Charles was announced in 1981, when she was 19, and the couple married that same year, going on to having two children — Princes William and Harry.

In July 1996, however, the couple divorced, and Diana was killed, along with her lover Dodi Fayed, in a Paris car crash in August 1997.

In a letter written in 1978 to Mary Clarke, her nanny, when she was 17, she wrote of how she was, along with her sister Jane and brother Charles, trying to "marry off (elder sister) Sarah to various men but have failed in every direction."

"We even thought Prince Charles might have been in the running but were relieved when that passed."

In the same letter, Diana lamented to Clarke that though she loved to sing and dance, "my voice sounds awful and watching me dance is like watching an elephant, so no one does!"

A separate letter written in January 1982, while pregnant with Prince William, Diana wrote of how "I am still feeling and being ill, well into my fourth month and am at total despair and long for the day when I can eventually sit on the loo as to looking over it!"

She continued: "I adore being married and having someone to devote my time, however, I do get annoyed at not being able to do my washing and general ironing."