CLICK ON PHOTO FOR FULL GALLERY OF WILLIAM AND KATE'S BEST SPORTING MOMENTS
"I would like to wish all of you in the England squad the best of luck in your game against New Zealand on Saturday," wrote William.
"We all know, of course, that the Kiwis will be brutally tough opponents probably the toughest you have faced in the tournament so far," he added.
"I am only sorry I cannot be there in person, but I will be watching and to you all, from Catherine and me, the very best of luck!"
Sporty duo William and Kate are known rugby fans, having often been spotted in the stands at matches.
Even Prince George has been given his own mini rugby shirt.
Wales' team captain Sam Warburton presented the shirt to William when the Duke dropped by the locker room before the Wales vs South Africa final of the Prince William Rugby Cup.