Oliver Fallon, who impersonated a postgraduate history student to gain access to the Scottish Catholic Archives (SCA), yesterday admitted at Edinburgh Sheriff Court stealing 288 items worth 26,400.
The thief, who had launched similar raids on rare documents at other archives, made five visits in July 2006 to the centre which is based in Drummond Place, Edinburgh, and holds records dating back to 1177.
After requesting access to a range of documents, which were then brought into the reading room, Fallon ripped out pages and hid documents inside his notebook, which he then smuggled out at the end of the day.
As the reading room did not have a member of staff present, the thefts went unnoticed until Fallon admitted the crime to police, after they discovered documents from other archives at his home in London.
When informed, librarians at the archives checked the database to see what documents he had accessed and were disturbed to discover pages had been torn out and other items were missing altogether.
The SCA looks after administrative, financial and legal documents of the Church, and many items are letters sent to churchmen from overseas and are valuable primary sources from the early days of the British Empire and the colonisation of the US.
It is understood a number of the items stolen were early forms of stamps that were attached to the documents and pre-date the introduction of the first public stamp, the Penny Black.
Fallon, of London, yesterday pleaded guilty to stealing various historical documents and letters from the Scottish Catholic Archives between 1 and 18 July, 2006. He is due to be sentenced next month.
The court was told visitors who want to view archives at the SCA must be interviewed by an archivist and are only allowed to take a pencil and paper into the reading room where they are shown the documents.
Fallon went to the SCA on 13 July claiming to be a postgraduate student from a university in London and gave his details.
He returned the following day and on another three occasions. John Cook, fiscal depute, told the court: "The archives date back to 1177AD and include documents of the administration, finance and legal matters relating to the Catholic Church in Scotland.
"The accused ripped pieces from documents and placed them in his notebook before leaving. Some 288 of the files were missing or mutilated."
Once he was caught, Fallon sent a letter apologising and returned 14,325 worth of records. But the damaged documents needed almost 5,000 worth of repairs, while 132 documents were still missing. These are worth 12,000.
His solicitor, John Mulholland, said his client disputed the value of the theft, but admitted Fallon was already serving time in England for similar offences.
Mr Mulholland said: "He told English police about the theft from Edinburgh and also told them where they could find some of the records. He can't say what steps, if any, were taken to follow up the whereabouts of further items."
Sheriff Noel McPartlin deferred sentence until May, and told Fallon: "There may well be a custodial sentence at the end of this case."