Whilst imprisoned, Henry III agreed to all of the terms of Magna Carta. Dated March 1265, British Library, Harley MS 489 f4.
Transcript
[1] Henry by the grace of God King of England. Know that I have conceded to all our faithful people all the liberties written below, by our charter, which formerly we caused to be made for them in the ninth year of my reign (1225).
I hold these rights to be reasonable and acceptable, renew them, sealed with my current seal, and concede and confirm them on behalf of the kings of England, for ever more.