Sir Paul McCartney and Nancy Shevell want to marry "soon" in a "small" civil ceremony, the US multi-millionaire has revealed.
The New York businesswoman and the ex-Beatle, who have been dating for almost four years, could wed in a register office.
Nancy, 51, told the New York Post that after the wedding, although she would "love" the couple to live in the US, she would "probably" move to England.
The heiress told gossip columnist Cindy Adams that the proposal, in California, where the couple had been staying, was "a total surprise".
Sir Paul, 68, was shown larger stones but picked a "vintage 1925 Cartier engagement solitaire diamond" because he loves antiques.
Talking about the wedding, Nancy did not reveal whether it would be in Britain or the US, but said it would be "soon" and "small" adding: "Just our families.
"I don't know the exact date. And don't ask what I'll wear because how dressy do you get to stand before a Justice of the Peace in his chambers, which is exactly what we're going to do," she said.
Sir Paul and nancy started seeing each other in Long Island's Hamptons area in late 2007.
The singer-songwriter's first wife, Linda, died of cancer in 1998 and he divorced his second wife, Heather Mills, in 2008.
Before she started dating Sir Paul, Nancy was married for more than 20 years to lawyer Bruce Blakeman.