0919 GMT May 24, 2022
Mark Royden attacked the protective glass surrounding the manuscript at Salisbury Cathedral in southern England but was chased off by two American tourists and cornered by a group of stonemasons, AFP reported.
The 47-year-old was found guilty in January of attempted theft and criminal damage to the security case costing £14,466 ($18,295, €16,172).
Sentencing him to four years in jail, judge Richard Parkes said he had made a "determined attempt" to steal a document of "huge historical importance" to the world.
Prosecutors said Royden believed the Salisbury Magna Carta was a fake and that he came equipped to carry out the theft on October 25, 2018, with a hammer, gloves and safety goggles.
Before smashing the protective case, he turned a security camera away to avoid being recorded, and set off a fire alarm as a distraction.
Salisbury Crown Court was told Royden had 23 previous convictions covering 51 offences, including theft and criminal damage against items "of the establishment".
His lawyer, Nicolas Cotter, said his client had a serious car accident in 1991 that left him with brain damage, and was a "caring, kind and helpful man" but was "blighted by demons".
The lawyer said Royden told him he could not plead guilty to the offence as it could "implicate him as a Russian spy".
Four original copies of Magna Carta from 1215 remain in existence: Two in the British Library in London; one in Salisbury; and another in Lincoln Cathedral, eastern England.
In June 1215, the despotic king John accepted the demands of rebellious barons to curb his powers and agreed the charter at Runnymede, a meadow by the River Thames west of London.
Copies were written out and sent around the country.
Magna Carta is considered the cornerstone of freedom, modern democracy, justice and the rule of law.
It has formed the basis of legal systems across the globe, Universal Declaration of Human Rights and the US constitution.
It declares that justice should be available to all, the law applies equally to all and leaders can only exercise power in accordance with the law.
Judge Parkes told the jury after they returned their verdict it was ironic the document Royden tried to steal enshrines an individual's right to "the lawful judgment of his peers".
"It still holds good and is in the process of the court right now," he said.