The large, 23 cm (9 inch) slice of cake icing, with a marzipan base, came from one of 23 official wedding cakes and was given to Moyra Smith who worked for the Queen Mother at Clarence House.

It was kept wrapped in clingfilm in an attic for the past 27 years.

Auction house Dominic Winter Book Auctions, based in Gloucestershire, described the white icing as having a sugared onlay of the royal coat-of-arms coloured in gold, red, blue and silver, a small silver horseshoe and leaf spray.

The cake was sold on Wednesday to an unnamed bidder along with a typed letter signed by "Charles and Diana" thanking Mrs Smith for her contribution towards a clock they were given as a wedding present.

The royal couple divorced in 1996.

"In view of its size, it is most likely that it was either from the side of a cake, or from the top of a single-tier cake. A highly unusual (and probably inedible) collector's item," read the description of the cake.

The auctioneer said the late Mrs Smith, a Scottish woman, worked at Clarence House, first in the kitchen and then moving to more general duties, and it was here that she was given the cake which was put up for sale by her family.

Her husband Donald told British media that his wife died last month, aged 78, and it was her wish for the cake to be sold after her death, with the money raised to go to charity.

The auction house also sold three autographed letters from Diana, who was killed in a car crash in Paris in August 1997, for between 740 pounds and 620 pounds ($1,350-$1,135).

The most expensive one was dated December 17, 1990 and addressed to Eileen and Tracey.

It read: "I was so very touched by all the lovely bottles that were in my Christmas bag today! Thank you both a million times for thinking of me. 1990 was the year that we met. At last, (!) & how wonderful our friendship was (& is) been. With special thanks & fondest love to you both."