Get ready for the wedding of the century -- and the making of a new British Princess!

After eight years of dating and two brief splits, Prince William and love Kate Middleton will finally tie the knot next year, multiple sources confirm to the new Us Weekly, out today.

"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.

"William told Kate she was The One, but he was not ready to get married. She...agreed to wait for him."

Adds another insider: "He wants to spend the rest of his life with her."

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.