Edmund soon clashed with King Henry III of England, defending church rights and criticizing the king's continental policies. Reacting against Henry, the baronage ultimately supported Edmund. Civil war threatened. Edmund forced Henry (by threat of excommunication) to expel his wife's ambitious French relatives and allies from England and to abandon their projects. Edmund further made Henry promise to observe English law, customs, and the counsel of his native magnates. In 1236 Henry requested the pope to send him a legate, Cardinal Otho, who arrived the following year. Otho's presence helped to undermine the archbishop's power. Henry upheld the monks of Canterbury in their opposition to Edmund's authority, and Otho's silence on that issue failed to help the archbishop. Edmund finally protested before Henry and generally excommunicated all who had infringed upon the liberties of his primal see. He left for Rome, planning to appeal his case before the Curia, but poor health forced him to stop at Soisy, where he died. He was buried at Pontigny Abbey. His admirers demanded his immediate canonization, which Henry opposed until 1247. Although Edmund had been made head of the English hierarchy in a crisis for which he was not prepared, the purity of his motives and the loftiness of his ideals commanded universal respect.

Of the various writings that are ascribed to Edmund, those assuredly authentic include Speculum ecclesiae (Eng. trans. by F.M. Steele, 1905), a widely known devotional treatise considered a major contribution to medieval theology.