The comedian was due to appear on the show with his partner Ben Miller.
However, The Sun reports that he found out he was about to become a dad for the third time while he was in his dressing room.
Miller went ahead with his appearance and told hosts Phillip Schofield and Holly Willoughby: "It's really exciting - Hannah called Xander and he just sort of said, 'I should go now'... I think it's three weeks early, but so was their last one, he said."
Armstrong and his wife already have two sons, Rex and Patrick.