These dresses are part of a sale of the late Princess of Wales' most valued items, including letters, accessories, and keepsakes. The collection also includes over 200 items from members of the Royal Family, which will go up for an auction in Los Angeles.
The auction titled: Princess Diana's Elegance & A Royal Collection, will take place at The Peninsula Beverley Hills and is organised by Julien's Auctions.
This is the most extensive collection of Diana's personal items to go under the hammer, since she sold dozens of her dresses at a charity auction in New York in 1997 before her death.
One of her gowns was a midnight blue tulle strapless gown, made by designer Murray Arbie. Princess Diana wore this dress twice in 1986, to the Phantom of the Opera premiere, and to dinner with King Constantine of Greece. She also wore it to a performance of Cinderella at the Royal Opera House in 1987.
A top estimate for this dress stands at $315,000 ($400,000).
The second gown is a magenta silk and lace off-the-shoulder evening dress, which was designed by Victor Edelstein. It was worn by the late Princess of Wales in London and Germany in 1987.
"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", said Juliens Auctions' co-founder and executive director Martin Nolan.
Diana was praised by Nolan as a "princess of economics", who wore gowns for not just environmental reasons, but to also "use her celebrity status to shine a spotlight on up-and-coming or new or unknown designers".
Such designers include Edelstein, Catherine Walker, and Caroline Charles.
Other items going under the hammer at tomorrow's auction include a Walker created pink floral silk shirt dress which Diana wore to a sports day in 1988 for her sons Prince William and Prince Harry. She also wore it when Prince Harry left school in 1992. The dress has a top estimate of £158,000 ($200,000).
Over 20 handwritten letters by Diana, as well as notes and holiday cards will also feature at this auction, described as showing "the kindness and concern that Diana had and also tells us she was truly happily married", according to Nolan.
