High Quality Content by WIKIPEDIA articles! Arthur I (29 March 1187 – ? April 1203) was Duke of Brittany between 1194 and 1202. He was the posthumous son of Geoffrey II, Duke of Brittany (d. 1186) and Constance, Duchess of Brittany. In 1190 he was designated heir to the throne of England and its French territory by Richard I; the intent being that Arthur would succeed Richard — in preference to Richard`s younger brother John Lackland. Richard, Geoffrey and John were sons — third, fourth and fifth, respectively — of King Henry II of England and Eleanor of Aquitaine.