On 28 September 1176, King Henry II betrothed Isabella to his youngest son, John Lackland.[1][2] John and Isabella were half-second cousins as great-grandchildren of Henry I, and thus within the prohibited degrees of consanguinity. But in the marriage agreement, the King agreed to find the best husband possible for Isabella should the Pope refuse to grant a dispensation for the marriage. Henry also declared Isabella the sole heir to Gloucester, disinheriting her two sisters.[2]
On 29 August 1189, John and Isabella were married at Marlborough Castle in Wiltshire, and John assumed the Earldom of Gloucester in her right.[2][3] Baldwin, Archbishop of Canterbury, declared the marriage null by reason of consanguinity and placed their lands under interdict. The interdict was lifted by Pope Clement III. The Pope granted a dispensation to marry but forbade the couple from having sexual relations.
Shortly after John acceded to the throne in 1199, and before the end of August, he obtained an annulment of the marriage. The annulment was granted on the grounds of consanguinity, by the bishops of Lisieux, Bayeux, and Avranches, sitting in Normandy. John, however, kept her lands, and Isabella did not contest the annulment.
After the annulment, John granted the title of Earl of Gloucester to Isabella's nephew Amaury, count of Evreux. This compensated Amaury for the loss of his French title, which was surrendered in the Treaty of Le Goulet. Upon his death without issue in 1213, Isabella once again became Countess of Gloucester.