A man who tried steal an original copy of Magna Carta from an English cathedral has been found guilty of criminal damage and attempted theft.

Key points: Royden had denied attempting to steal the document from Salisbury Cathedral

Royden had denied attempting to steal the document from Salisbury Cathedral The court heard he set off a fire alarm in the cathedral and left three holes in the casing with a hammer

The court heard he set off a fire alarm in the cathedral and left three holes in the casing with a hammer He tried to flee the cathedral but was grabbed by maintenance workers and visitors

Mark Royden, 47, smashed a glass box housing the priceless medieval manuscript in Salisbury Cathedral in southern England in October 2018.

Salisbury Crown Court was told he set off a fire alarm in the cathedral cloisters before hitting the glass case with a hammer, smashing three holes in it and causing damage estimated at 14,000 pounds ($27,300).

Failing to break through the safety glass, he tried to run out of the cathedral but was grabbed by maintenance workers and visitors.

The parchment, one of only four original copies of the 1215 charter still surviving, was not damaged and no-one was hurt in the incident.

Royden smashed three holes into the glass case with a hammer. ( Supplied: CPS )

Magna Carta is considered to be one of the most important documents in the history of democracy and a key manuscript in English history

It is a charter of citizens' rights, curbing the arbitrary power of medieval kings and guaranteeing the right to a fair trial.

King John agreed to place his seal on the document in June 1215 at Runnymede, near Windsor, west of London, in an attempt to end an uprising by rebel barons.

Prosecutors said that when questioned by police Royden had appeared to question the authenticity of the document.

"The historical importance of the Magna Carta in establishing the right to justice cannot be overstated, which is somewhat ironic given Mark Royden's repeated denials of his crime in the face of overwhelming evidence," said Rob Welling of the Crown Prosecution Service (CPS).

"Had he succeeded in taking it, Royden would have deprived the nation of what is said to be the most beautiful surviving copy from 1215.

"Fortunately for the rest of us, he spectacularly botched this theft — though not before causing 14,000 pounds' worth of damage."

The document, one of four remaining original copies of Magna Carta, is back on display at the cathedral. ( Reuters: Kieran Doherty, file )

A statement on the CPS website said CCTV footage showed Royden changing the angle of the camera lens closest to the display to prevent it capturing his attempt.

It also said Royden's discarded hammer had shards of security glass on it. Royden told police the tool might have some of his DNA on it because it had brushed up against him.

He was released on bail and will be sentenced on February 25.

The document is back on display in Salisbury Cathedral.

ABC/Reuters