During the course of the parliament held between January and March 1265 by the revolutionary council led by Simon de Montfort, earl of Leicester, Henry III confirmed Magna Carta. Although the significance of Montfort’s parliament has been debated throughout the intervening centuries, its role in the story of Magna Carta has generally been forgotten. This is probably because it is not generally appreciated that the text of the 1265 confirmation of Magna Carta still survives. The confirmation was not enrolled on the Charter Rolls and no engrossments have been discovered. Yet the text has been preserved in four copies, three of them giving the witness list detailing those who endorsed the confirmation. It supplies extremely valuable evidence for the nature of the Montfortian support base at this momentous parliament, as well as for the strategies that Montfort’s council pursued to entrench its authority. It is the purpose of the present article to bring attention to this little-known text, to make its contents more accessible by providing a transcription and translation and to discuss its context and significance.