The dresses, worn by the late Princess of Wales during the 1980s, are the most highly-valued items in a collection that includes some of her letters, accessories and keepsakes as well as more than 200 items from members of the Royal Family, going under the hammer in Los Angeles.

Julien's Auctions has called it the most extensive collection of Diana's personal belongings to be auctioned since she sold dozens of dresses during a New York charity auction in 1997, shortly before she died.

A pair of gowns sold during the New York auction, two months before her death in a car crash in Paris, top the upcoming sale, titled Princess Diana's Elegance & A Royal Collection, at The Peninsula Beverly Hills.

Designer Murray Arbied made the first, a midnight blue tulle strapless gown that Diana wore twice in 1986 - to the Phantom Of The Opera premiere, to dinner with King Constantine of Greece - and to a Royal Opera House performance of Cinderella the following year.

It has a top estimate of $400,000 (£315,000), as does a magenta silk and lace off-the-shoulder evening dress designed by Victor Edelstein, which Diana wore in London and Germany in 1987.

Martin Nolan, co-founder and executive director of Julien's Auctions, said: "People will know that name because Victor Edelstein also designed the very famous navy blue dress Diana wore when she danced with John Travolta at the White House in 1985."

Diana, he said, was a "princess of economics," wearing gowns numerous times not only for environmental reasons but also to "use her celebrity status to shine a spotlight on up-and-coming or new or unknown designers," including Edelstein, Caroline Charles, and Catherine Walker - all of whom feature in the auction.

One of Walker's creations, a pink floral silk shirt dress Diana wore to a 1988 sports day for Prince William and when Prince Harry left school in 1992, has a top estimate of $200,000 (£158,000).

In December a Jacques Azagury-designed dress worn by Diana in Italy in 1985 became the most expensive dress worn by the royal at auction, selling for more than $1m (£790,000).

More than 20 of Diana's handwritten letters, notes, and holiday cards are also up for grabs in this week's sale.

The keepsakes show "the kindness and concern that Diana had and also tells us she was truly happily married," Mr Nolan said.

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Many of the letters are to Maud Pendrey, a former housekeeper at the Spencer family home at the Althorp Estate in Northamptonshire, where Diana grew up.