The couple met as undergraduates in Scotland and had been dating for nearly eight years. They announced their engagement in November.
The highly anticipated ceremony is set for April 29. Britons have been given the day off to watch.
Fresh pictures have emerged of Middleton attending a wedding - this one of her friends Harry Aubrey-Fletcher and Louise Stourton, held on Saturday in the northern England village of Aldborough.
The bride-to-be wore black, sporting a velvety topcoat over a black dress and a black pillbox hat. Prince William, who also attended, wore a traditional dark morning suit, and managed to joke with photographers after the ceremony.
"Hello, hello," he said, jumping onto the sidewalk amid the camera flashes. "It's not my wedding yet."
The Associated Press contributed to this story.
