The A-list couple, who are close pals with Queen Elizabeth's granddaughter Princess Beatrice and her boyfriend Dave Clark, are reportedly eager for a royal affair of their own after falling in love with Britain, where Mila recently filmed new movie Jupiter Ascending.
A source told the Daily Star newspaper: "They both adored Kate and William's wedding and wanted to do something similar for themselves. They don't want a big flashy wedding but they liked the idea of marrying somewhere historic like a castle.
"But then Princess Beatrice suggested the Orangery at Kensington Palace and Mila and Ashton think it's the perfect setting.
"They want a small ceremony in front of just a few family and friends."
However, the loved-up couple will have to wait until Ashton, 35, is officially divorced from ex-wife Demi Moore before they can tie the knot.
The Two and a Half Men star split from his estranged spouse in 2011, but their divorce has been held up by disagreements over settlement money. The former flames are thought to have reached an understanding recently and agreed to sign their divorce papers.
As well as marrying, Ashton and Mila, 30, are looking to start a family and settle down.
The insider claimed: "They would love to start a family, but they've both agreed they'll wait until they're married.
"Ashton and Mila are so looking forward to having children. Their first baby might even be conceived in the royal residence on their wedding night."
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