Such generous grants were not surprising for a royal cousin who posed no threat to the throne and as his sister Adela de Warenne was one of John's mistresses.

[6] During the king's difficulties with the barons, when they sought for the French prince to assume the English throne, William is listed as one of those who advised John to accede to Magna Carta.

His allegiance only faltered a few times when the king's cause looked hopeless; shortly before the death of John, he made terms with Prince Louis.

In March 1217 he again demonstrated his loyalty to England by supporting the young King Henry III, and he was also responsible for the establishment of Salisbury Cathedral.

In 1238 he was evidently regarded as a leader of the baronial opposition, for the Great Council appointed him as one of the treasurers who were to prevent the king from squandering the subsidy voted in that year.