Briege Mackenzie recently picked up the little black number in a New York auction house, but has since realised she cannot quite squeeze into it.
The trouble is that the frock is second-hand, and the previous owner is svelte Diana, Princess of Wales.
The dress, which Diana wore to the Serpentine Gallery on the night the Prince of Wales went on television and admitted adultery, is a size 10.
Forty-four-year-old Mrs Mackenzie however is a couple of sizes larger. But the mother-of-three, of Bridge of Weir, Renfrewshire, has no plans to go on a crash diet.
"I am not some rich bitch pretending she's a princess by buying one of her dresses to prance around cocktail parties in.
"I can't fit into it, but that's not really the idea in this instance. We bought it to raise funds for the charity Children 1st," she said.
Mrs Mackenzie and her husband Graeme, thought that by purchasing one of the dresses they could benefit their favourite charity.
"We haven't had time to work out any details of how it will be used but, at least for the next year, it will certainly raise funds for Children 1st. Then we may allow it to be used by other good causes," said Mrs Mackenzie.
Children 1st is a Scottish charity which aims to prevent neglect and abuse against children.
Barry Ayre, the charity's director of fund-raising, said: "We are absolutely delighted. Briege Mackenzie has been a long time supporter of Children 1st and we are very grateful. We are going to plan a series of fashion shows."Reuse content