Prince Charles and Princess Diana tied the knot at St Paul's Cathedral 40 years ago tomorrow. Their nuptials were widely described as the wedding of the century and a fairytale ceremony.

       While the union of the Prince and Princess of Wales broke down a few years later and ended in divorce in 1996, a slice of the wedding cake has survived the test of time and is now up for auction.

       The slice measures 8ins by 7ins and weighs 28oz.

       The slice, made of icing and a marzipan base, presents an almost intact royal coat of arms coloured in white, blue red and gold.

       It has been preserved wrapped up in cling film inside an old cake tin decorated with pink flowers.

       On the cover of the tin, there is an old note dated July 29, 1981 reading: "Handle with care. Prince Charles and Princess Diana's wedding cake."

       It also presents the signature of M. C. Smith - presumably the person who received the cake after the wedding.

       The slice was given to Moyra Smith, one of the Queen Mother's employees at Clarence House.

       It remained with Ms Smith's family until 2008 and was then acquired by a collector.

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       They are now selling the slice with auctioneers Dominic Winter of Cirencester, in Gloucestershire.

       The slice is believed to have been cut from a cake distributed to staff at Clarence House to thank them for their efforts.

       A total of 23 official cakes were made for the wedding.

       Prince Charles and Diana's main wedding cake was a 5ft tall layered fruitcake treat, weighing approximately 225lbs.

       This slice of cake will be sold on August 11 alongside an order of service for the wedding and the Royal Wedding Breakfast programme.

       Described by the auctioneers as a "destined to last" memorabilia, the cake slice has been tipped to go for ￡500.

       The sellers are advising against tasting the cake for health reasons.

       Chris Albury, specialist Royal memorabilia valuer at Dominic Winter, said: "I still wouldn't recommend eating it but after 40 years it's clearly destined to last.

       "It's a curious and unique keepsake celebrating a royal wedding that holds an enduring fascination with British royalty aficionados worldwide."

       This slice isn't the first from one of Charles and Diana's wedding cakes to be sold at auction.

       In 2014, another slice was sold during an auction in Los Angeles for ￡990 ($1,375).

       It was still wrapped in its original wax paper and preserved in a white and silver cardboard box reading "Buckingham Palace 29 July 1981".

       This slice of cake waiting to be auctioned isn't the only memento from the royal wedding of Charles and Diana to have recently resurfaced.

       Princess Diana's historic wedding dress has been loaned by Prince Harry and Prince William for the ongoing exhibition at Kensington Palace 'Royal Style in the Making'.

       The dress was designed by David and Elizabeth Emanuel and features a 25ft sequin-encrusted train.

       In a video shown at the exhibition, Ms Emanuel remembers being asked to make the dress.

       She said: "It was one of those strange moments where you know your life is never going to be the same again."