There are three other authentic examples of the Magna Carta
Salisbury Cathedral's copy of the Magna Carta is being hailed as the best in existence.
The 1215 copy of the document, which is displayed in the cathedral's chapter office, has been added to Unesco's Memory of the World register.
It said the Salisbury copy was in "very good condition" despite the cords and seal being lost.
Canon Chancellor Ed Probert said a previous canon was at Runnymede in 1215 and may have brought it back.
"One of the key people in the service of the Archbishop of Canterbury who was responsible for the agreement of the Magna Carta was a canon of this cathedral called Elias of Dereham - thought of as a probable architect of the cathedral at Salisbury.
"He was at Runnymede and and may well have brought the copy back with him," he added.
There are three other authentic examples of the Magna Carta, two held by the British Library, the third in Lincoln Castle.
The Magna Carta forms part of the common law of England and its content defended, maintained or repealed as the needs and functions of the law have required.
Each of the four copies is unique but none has more authority than any other
It is believed they have been written on parchment in some haste by different Chancellery clerks but the sizes and texts differ slightly in presentation.