Arthur I, Duke of Brittany (1187 – 1203), was the posthumous son of Geoffrey Plantagenet and Constance, Duchess of Brittany, and designated heir to the throne of England, originally intended to succeed Richard I.

While Richard was away on crusade, Constance took more independence for Brittany, and in 1194 had the young Arthur proclaimed as its Duke.

When Richard died in 1199, his brother John immediately claimed England, but much of the French nobility refused to recognize him as king, preferring Arthur, who declared himself vassal of Philip Augustus. This was sufficient excuse for John, who invaded France in 1202.

Philip having recognized Arthur's right to Brittany, Anjou, Maine, and Poitou, Arthur invaded the last-named. But on July 31, 1202 he was surprised by King John while besieging Mirabeau (where he was holding his grandmother Eleanor of Aquitaine hostage), captured, and imprisoned at Falaise, guarded by Hubert de Burgh. At this time also his sister Eleanor was captured and imprisoned at Corfe . The following year he was transferred to Rouen, under the charge of William de Braose, and then vanished mysteriously in April 1203.

The puzzle of his disappearance gave rise to various stories. One account was that Arthur's jailors feared to harm him, and so he was murdered by John directly and his body dumped in the Seine. William de Braose did rise high in John's favor after Arthur's disappearance, so much so that he was suspected of complicity, and indeed many years later, after difficulties with John, William's wife Maud de Braose directly accused the king of murdering Arthur, which resulted in Maud and her eldest son being imprisoned and starved to death therein. William escaped to France, where he was supposed to have published a statement on what happened to Arthur, but no copy has been found.

References

• M. Dominica Legge, "William the Marshal and Arthur of Brittany", Historical Research, volume 55 (1982)
• F.M. Powicke, "King John and Arthur of Brittany", The English Historical Review, volume 24 (October 1909), pp. 659-674