SOURCE: BBC (3-27-06)
Although not dated, experts know the charter was made on 3 September 1101, more than 900 years ago, and have confirmed that it is original.
The castle's owner John Berkeley said it had lost its seal, but was in all other ways authentic.
"I'm amazed that such an ancient document has turned up among my family papers," he said.
"Our archivist, David Smith, came across it while looking for something else. I had no idea it was there."
Only about 300 such documents from Henry I's reign (1100 to 1135) have survived.
An extract, translated from the Latin, reads: "Know that I have given to God and to St Peter of Gloucester and Serlo their Abbot for the sustenance of the monks my land of Maesmores and all the woods and fields."
Mr Berkeley said the document was found amongst papers relating to the family's Spetchley Park property in Worcestershire.
"It's especially interesting... because this year we are celebrating the 400th anniversary of its purchase by my ancestor Rowland Berkeley on 30 May 1606. It was when Mr Smith was looking for that deed that he found the charter," he said.