The slice was sold on behalf of Rosemarie Smith, whose daughter worked as a maid and a dresser for the royal family for almost three decades.
"There were two telephone bidders fighting it out for the slice. It was exciting," a spokesman said, adding that the toast was sold for 230 pounds (USD 361).
Rosemarie Smith snatched the crispy piece of bread during a visit to Buckingham Palace. Her daughter had invited Smith to spend the hours ahead of the wedding at the palace with her.
"At the time my daughter was a maid at the Palace and one of her duties was to collect Prince Charles's breakfast tray from outside his room," Smith said on the website of Charles Hanson Auctioneers.
"I was with her in the corridor and saw that Prince Charles had left some toast on the tray. I had been thinking about a keepsake from the wedding and saw the toast and thought to myself: 'Why not'?" Smith said.
William and Kate's wedding and the Queen's Jubilee got Smith thinking that her bit of royal bread could be worth something.
"I just wandered into the auctioneers out of curiosity and asked them if it was worth anything," she said, adding: "I was pleasantly surprised to hear them agree with me that it could be of quite some value to Royal collectors."
