The life and times of a 5th Century Cornish monk has sparked an unholy row over a manuscript in verse.
The monk is said to have sailed to Cornwall on a floating boulder
St Ke or Kea is reputed to have helped King Arthur solve a family dispute.
But a play about him written in medieval Cornish has sown discord between the National Library of Wales and the Cornish Language Board.
The library is still working on a translation of the verses kept in its vaults. But the board printed its own version, and says it is "selling well".
The copyright dispute centres on a previously unknown 16th Century manuscript bequeathed to the library in 2000 by the late Professor JE Caerwyn Williams, an expert in Celtic languages.
The 40-page manuscript, based on an earlier copy, is written in rhymed verse divided into stanzas, with stage directions in Latin, and is published on the library's website.
Legend has it that St Ke or Kea was an Irish-born monk who sailed to Cornwall on a floating boulder, landing near Truro, where he founded a church.
A Latin text reported he was summoned from Brittany to sort out a conflict between King Arthur and his nephew Modred.
The Cornish Language Board said it had published 200 copies if its work, Bywnans Ke (Cornish for The Life of St Ke) at £11 each.
The language board says its book is "selling well" among the country's 500 fluent Cornish speakers and 3,000 Cornish users.
Cornish language enthusiasts said they had become impatient waiting for the library to translate the work, so they went ahead with their own version.
A tower stands at Old Kea, Cornwall, where St Ke is said to have landed
One, who did not want to be named, said: "There is quite strong feeling that this is Cornish heritage and we should have it.
"They are saying they have the copyright. We are saying we have not published the manuscript but a reading and a translation of it.
"It's not like sitting down and reading a newspaper, someone has to spend time studying it and reading each individual word and working out the what exact spelling is and then deciding what the words are."
Another, who lives in Wales, said: "How can they have copyright over something that was written hundreds of years before the laws were drawn up?"
George Ansell, of the Cornish Language Board, declined to comment, adding that the body was seeking legal advice about the national library's letter.
The national library said it had written to the language board seeking answers to questions about the book's publication and hoped for an "amicable agreement" as soon as possible.
A spokesperson said: "The library is eager to promote all things Celtic and is pleased to co-operate with other Celtic institutions whenever possible in order to further the cause of Celtic languages.
"The library has been working towards publishing the Cornish manuscript for a number of years and in co-operation with other partners.
"Unfortunately the library was not notified by the Cornish Language Board of their intention to publish, although the board knew of the library's plans."