A slice of Princess Diana's wedding cake, kept for 27 years in a former servant's attic, could fetch as much as £20,000 when it is sold at auction today.

The nine-inch square piece of confectionery was given to cleaner Moyra Smith in 1981 when she worked for the Queen Mother. Moyra died last month aged 78 and it was her dying wish for it to be sold.

Wrapped up in a metal tin the slice has remained in the loft of the London home of Moyra and her husband Donald ever since.

Icing on the cake: The 27-year-old cake from the late Princess of Wales' wedding is expected to fetch up to £20,000 at auction

The cake, along with a personally signed letter to Moyra from Diana, will go under the hammer at specialist auctioneers Dominic Winter Book Auctions, South Cerney, Wilts.

In 1998 a similar slice sold fo £17,000, a year after the princess died in a Paris road crash.

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The slice of cake, the signed thank you letter and a bottle of beer produced to mark Prince William's birth in 1982 are in the lot

Widower Donald said his wife had a special place in the hearts of the Royal Household.

He said: 'Moyra was an amazing woman.

'I'm not as sentimental as she was was about these sort of things, but I would be nice to see the cake go to a good home.

'I heard the last slice went for a fortune so that would be great for our charity.

'Before she died Moyra told me that she had put some money away for me and our daughter Carolyn, but I didn't realise she had kept this for so long.

'She was always putting away money here and there, I found £100 in old pound notes as well.

'I found the cake in the loft, still wrapped in the same clingfilm she had put it in when she brought it home.'

The Princess and Prince Charles at their wedding in 1981

Donald, who would have celebrated 50 years of marriage to Moyra this year, said the find had brought back happy memories of his wife's time with the Royals.

He said: 'She was very popular among the staff at the house.

'She worked there for five years in all, from 1981 to 1986, and we had some happy times.

'We were invited to several royal functions and the Queen Mother often asked after our daughter Carolyn, who was training to be a dancer.

'Diana was very kind when we met her.

'When the cake arrived at Clarence House Moyra was told she could have a slice from the top tier to keep for herself.'  Diana memorabilia specialist auctioneer Chris Albury, from Dominic Winter, said the cake was the biggest piece he had ever seen.

He said: 'This piece is special because it was from the cake sent to the Queen Mother at Clarence House.

'I have seen lots of smaller pieces before but never one from the Royal Household.

'Moyra must have been very well thought of by the Royal Family to have been given this slice from the top of the cake.

'Diana memorabilia holds its value very well because there are some avid collectors out there.

'The decorative sugar icing of the royal coat of arms on top of the cake is very skilled and while there is some cracking and damage it is in remarkably good condition.'

A personally signed letter from the princess thanking Moyra for her contribution to a wedding gift is also being sold in the lot.

Chris said: 'We are putting a value of at least £600 on the letter because it is signed by Diana herself.

'It has been personally signed by Diana to Moyra, which makes it very special.

'It thanks Moyra for her work in the house and her contribution to a wedding gift for Charles and Diana.'  Retired jeweller Donald, who will attend today's auction, hopes the sale of the cake will raise money for a charity set up by the couple to help the elderly.

He said: 'We helped start a charity call Furzedown which organises trips for elderly people in London.

'Moyra donated £2,000 for a minibus so I could drive groups for days out in Brighton and Bournemouth.'