The dress, which auctioneer Kerry Taylor described as "iconic," had been expected to sell for 30-50,000 pounds.
It was part of a collection of clothes being sold by designers Elizabeth Emanuel and David Emanuel and was bought by fashion museum the Fundacion Museo De La Moda in Chile.
"As the weeks before her wedding progressed, Diana lost so much weight that she returned the gown to the Emanuels to have it taken in," Taylor said in a statement.
"The alterations required were so drastic that they decided it would be easier to re-make a smaller version for her. They kept the original and forgot about it -- only re-discovering it quite recently."
Overall, the 30 lots on offer fetched 282,720 pounds ($407,000).
"The high prices that we saw today reflect the enduring love affair with Princess Diana," Taylor added.
Jorge Yarur, founder of the museum which bought the dress, said that when he died he planned to donate it and other Diana dresses he owned to Kensington Palace, where she lived from the time of her wedding in 1981 until her death in 1997.
The palace already displays a selection of Diana’s dresses in the Royal Ceremonial Dress Collection.