YOU ALREADY KNOW…that the whole world’s buzzing about “the dress” – asking: Who’ll design the princess gown for KATE MIDDLETON’s wedding to Britain’s PRINCE WILLIAM? What will it look like?

NOW HERE’S MY SCOOP: Thrilled as she was when Future Hubby pledged his troth with PRINCESS DIANA’s famed sapphire and diamond engagement ring, Kate was horrified, says a source close to the royals, when William blurted: “I want you to wear Mother’s wedding dress, too!”

Stunned, Kate listened in disbelief as William told her he’d arranged to have the dress, the centerpiece of a touring exhibition of 150 items from Diana’s life, rushed back to London.

Instantly, Kate told him: “NO!…Absolutely not. I cannot wear your mother’s wedding dress.” Making it clear she respects her future husband’s need to honor Mother’s memory, she explained gently: “It’s one thing to be wearing her ring, but NOT her wedding dress! It’s going too far.”

Seeing the sense of what Kate was saying, William backed off and apologized, said the source. “He told Kate, ‘I now realize that this must be YOUR special day.’ ”