"The engagement will be announced in 2011," a source says. (A palace rep tells Us it does not comment on the private lives of the couple.)
Yet the heir to the British throne and his commoner girlfriend, both 28, made a secret pact to marry three years ago while on vacation in the Seychelles. "Kate wanted assurances from William," Katie Nicholl, royal watcher and author of William and Harry: Behind the Palace Walls, tells Us.
For more on the royal nuptials -- why the chosen year 2011 will "honor" the late Princess Diana's memory, how Kate's mom "put some pressure on" the Prince to propose, what kind of gown Kate wants, and their life together now -- pick up the new Us Weekly, on newsstands today.