The William Wallace Letter is a formal note from King Philip IV of France to his agents in Rome, commanding them to recommend the Scottish warrior to the Pope.
Historians believe that the post-card-sized document was written in 1300 to help Sir William Wallace secure safe passage on a journey across Europe to Rome, and that it was confiscated when he was captured by English forces at Robroyston five years later - the arrest that would lead to his trial and execution for treason.
The letter was discovered in the Tower of London in the 1830s, and is stored at the national archives in Kew, in Surrey.
Little was knownabout it until the Society of William Wallace, an organisation dedicated to preserving his legacy, launched a campaign for its return to Scotland in 2005, as they marked seven centuries since his death.
"We do not have a lot of tangible links with Wallace, as most of the documentation has been destroyed," says Gary Stewart, vice convenor of the society.
"We felt it would be a massive boost for Scotland and for the tourist industry to have something that Wallace actually touched. For people to see this document and feel a connection to Wallace is something they could never get by seeing a copy of the letter on a computer screen."
Under the leadership of David R. Ross, the society's inspirational convenor, who died last year, members worked to raise awareness, generating media coverage and gaining support from the public and politicians, including Christine Grahame MSP, who lodged a parliamentary motion calling for the document's return, and encouraged the group to start a petition to help focus political and public attention on their demands.
Hopes that the Scottish Government would make a request for a transfer to Scotland faded in March 2009, when a response to a parliamentary question from Ms Grahame claimed that a lack of proof about the letter's provenance meant there were insufficient grounds for a request.
"In the case of the Wallace document in the national archive, the national archive of Scotland genuinely does not have enough evidence to substantiate a claim that it should be returned to Scotland." wrote George MacKenzie, Keeper of the Records at the National Archives of Scotland.
While government archivists in London and Edinburgh dismissed requests on the grounds that the document was an English copy of the original French script, the campaigners set out to prove that they were wrong - with the help of a Professor Geoffrey Barrow, the highly regarded Scottish historian.
"Professor Barrow showed us a book with examples of French script and English script of the time, and it became very clear that the writing on this document was French," explained Ms Grahame. "I was taken aback by how long there had been a wall, saying that provenance was not assured.
"Another objection was that there was no seal - but the letter had clearly been ripped, so it could have been removed. We also learned that the custom was for letters to be carried folded in a pouch, so it may never have been sealed anyway!
The campaign continued despite obstacles, and as the petition gained support, Fiona Hyslop, the Culture Minister, called for an independent academic research group to be assembled to consider the case.
They agreed it was likely that the document had been in William Wallace's possession.
By the time the petition was lodged on August 23rd 2010, its instigators had gathered 2577 signatures from supporters across Scotland, and as far afield as Spain, Austria, Canada and the United States.
Following its submission, and other representations on the matter, the Scottish Government put a written request to the Ministry of Justice for the William Wallace Letter to come to Scotland, and in September 2011 it was agreed that it would be transferred on long-term loan from January 2012 until at least December 2014.
Although the letter is due to arrive in Scotland next month, it will not be on display immediately, as it is so fragile that it can only be shown for a few weeks each year. It will initially be kept by the National Archives of Scotland in Edinburgh, who will announce plans for it to be exhibited, along with the Lubeck letter, written by Wallace, in the summer.
It is an arrangement that Christine Grahame is convinced would not have come about without the efforts of campaigners: "It just shows the power of a petition, and the power of an organisation whose members are determined and know their stuff. They taught me not to take no for an answer from the establishment if you think they are absolutely wrong."
Like the last letter of Mary Queen of Scots, which was recently shown at the National Library of Scotland in Edinburgh, Ms Grahame believes that the document will have the power to inspire those who see the original for themselves: "There's a magic about these artefacts because they have been in the hands of that person, that giant in Scottish history," she says.
Beyond 2014, the document's home remains unconfirmed, but members of the Society of William Wallace are satisfied that their campaign has been a success, as it will enable public access to an artefact that was previously out of reach.
"Without the petition, many people may not have got involved," says Gary Stewart, "it has enabled us to achieve a goal that the late David R Ross set, which means so much to all of us involved. We are grateful for all the help and support we have been given."
The William Wallace Letter: Translation
Philip by the grace of God, king of the French, to his beloved and loyal people appointed at the Roman Court, greetings and favour. We command you that you ask the Supreme Pontiff to consider with favour our beloved William le Wallace of Scotland, knight, with regard to those things which concern him that he has to expedite. Dated at Pierrefonds on the Monday after the feast of All Saints [7 November 1300].