A collection of gowns, accessories, and keepsakes belonging to Diana, Princess of Wales, is set to go under the hammer.

The auction is billed as the most extensive collection of Diana’s personal belongings since she sold dozens of dresses at a New York charity auction in 1997, according to Julien’s Auctions.

A pair of gowns sold during that auction, two months before her death, top the sale on June 27 at The Peninsula Beverly Hills in Los Angeles.

A Murray Arbeid midnight blue tulle strapless gown that Diana wore twice in 1986 — to the Phantom Of The Opera premiere and to a dinner with King Constantine of Greece — and to a Royal Opera House performance of Cinderella in 1987 has an estimate of 400,000 US dollars (£315,000).

A magenta silk and lace off-the-shoulder evening dress designed by Victor Edelstein, which Diana wore in London and Germany in 1987, has the same price estimate.

“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,” Martin Nolan, co-founder and executive director of Julien’s Auctions, told the PA news agency.

Mr Nolan described Diana as 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.

A pink floral silk shirt dress designed by Walker, which Diana wore to Prince William’s sports day at Richmond Stadium in 1988 as well as leaving Prince Harry’s school in Notting Hill in 1992, has a top estimate of 200,000 dollars (£158,000).

The sale comes after a Jacques Azagury-designed dress worn by Diana in Italy in 1985 set a new world record for the most expensive dress worn by the royal at auction, selling for more than one million dollars in December.

After that auction, Mr Nolan told PA that “the phones were ringing” as people realised “Diana was highly collectable, reaching record prices”.

“Even one of her friends from school, (one of) her lifelong friends that Diana used to give her shoes, bags and dresses to when she finished with them, she came to us, and she said ‘I’d love to sell these items because I know that Diana would want these items to go to good causes’,” Mr Nolan said.

“She’s raising money for Muscular Dystrophy UK, which is a charity that’s very important to her.”

The forthcoming sale, held by California-based Julien’s Auctions, features more than 200 items from members of the royal family.

“But of course Princess Diana takes centre stage with over 50 items from her life and career, incredible gowns from all her favourite designers, but also very personal handwritten notes and cards,” Mr Nolan told PA.

The auction features more than 20 handwritten letters, notes, and holiday cards from Diana, which “really speaks to the kindness and concern that Diana had and also tells us she was truly happily married”, he said.

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

“She’s writing to Mrs Pendrey about the honeymoon (being) a huge success and pregnant with William, and then William was born and then pregnant with Harry and Harry was born,” Mr Nolan said.

“We sort of forget that she was truly happy in her marriage in those early days, and these letters tell us that, so it’s an important part of our royal history as well.”

Mr Nolan added that Diana is “still loved dearly today” as “the mother of the future King of England”.