London, Sep 17 - The younger sister of Queen Elizabeth II destroyed letters from Princess Diana in a bid to 'protect' the British royal family, a new book reveals.

The late Princess Margaret told a friend that she had ordered the destruction of large bin bags full of personal papers 'because they were so private', says the book - a biography of Queen Elizabeth II's mother, who died in 2002 at age 101.

The book, 'Queen Elizabeth the Queen Mother: The Official Biography', by journalist William Shawcross, says Princess Margaret went through her mother's papers in 1993, several months after Prince Charles and Diana had announced their separation.

Journalist Andrew Morton's 1992 book 'Diana: Her True Story' had disclosed that the Princess felt she had been rejected not only by Charles but also by the Queen and other members of the Royal family.

Although it was reported that Margaret had burned bundles of letters to her grandmother, this is the first confirmation that Diana's letters were among them, the Daily Telegraph newspaper reported Thursday.

In the book, commissioned by Queen Elizabeth II, Shawcross writes: 'Princess Margaret was engaged on one of her periodic 'sortings' of her mother's papers.