The road from a Thameside meadow in the 13th century to a New York auction house in 2007 is a long one, but it’s the route Magna Carta has taken. For the first time in history the great charter is going under the hammer.

David Redden, vice-chairman of Sotheby’s, the auctioneers, calls it “the most important document in history”. Bids are expected to reach between $20m (£9.8m) and $30m (£14.7m) when it goes on sale on December 18.

“This was the birth certificate of freedom, the foundation of law in the United Kingdom, the US, and throughout the world,” Redden said.

The story of how the barons forced King John to sign Magna Carta at Runnymede, a meadow by the Thames west of London in 1215 is part of English mythology. In fact, their deal concentrated on the rights of the nobility and lasted only a few months. It was not until 1297 that a revised charter became law – and it’s a copy of this version that is for sale.

Ross Perot, the Texan billionaire and former presidential candidate, bought it in 1984 from the Brudenell family of Deene Park, Northamptonshire, who had owned it for about 500 years. It is the only copy in private hands. The Australian government owns a copy and others are held at the British Library (along with the original) and in Lincoln and Salisbury cathedrals.

This is not just a question of history, however. Although Magna Carta is about to be dropped from the English school curriculum, it remains highly relevant to one of our great political controversies today: the row over the government’s efforts to extend the number of days that suspects can be held by the police without charge.

Clause 39 of Magna Carta states: “No free man shall be taken or imprisoned or disseised or outlawed or exiled or in any way ruined, nor will we go or send against him, except by the lawful judgment of his peers or by the law of the land.”

Ever since Tony Blair proposed extending the period of detention for suspected terrorists to 90 days two years ago, his critics have warned that immemorial laws and ancient freedoms are under threat – and have succeeded in defeating not just 90 days but 50 days.

Jacqui Smith, the home secretary, is now suggesting 42 days. But for opposition parties, civil liberties groups and some Labour backbenchers even the current 28 days is pushing it.

Historically, it has been in times of war, when national security is under threat, that detention without charge and the curtailing of habeas corpus have been invoked. Clearly we are again immersed in one of the moments in history when circumstances are forcing us to reexamine our ideas of freedom.

The times in which we live are unique if you look beyond security and terrorism to questions of privacy and identity fraud that are symptomatic of our digital age.

The furore over detention presents Gordon Brown with possibly his greatest challenge yet as prime minister. But it might prepare him for the fight he will also have on his hands over his desire to rethink the constitution.

Recently, Brown used a speech on liberty at Westminster University to announce plans to bring about these reforms. As the original Magna Carta was introduced to limit the powers of the monarch, his constitution would limit the powers of government.

“In a world of increasingly rapid change and multiplying challenges,” he said, “democracies must be able to bring people together, mark out common ground, and energise the will and the resources of all.”

This goes beyond a collective concept of liberty to the theme of “Britishness” that Brown has warmed to – and to his ambitions for a “Britain Day”, which brings us back to Magna Carta.

In an attempt to decipher what Britain Day might celebrate, BBC History magazine conducted a poll of its readers. The anniversary of the signing of Magna Carta emerged triumphant, leaving VE Day and D-Day in the shadows.

Dave Musgrove, the magazine’s editor, said: “It’s fascinating, and surprising, that an event from medieval history has come out above VE Day, all the more so when you consider that it’s a constitutional rather than a militaristic moment that’s been chosen.”

History buffs do not necessarily speak for the nation on this one, and a more extensive poll might put VE Day top. According to Chris McGovern, a headmaster and historian: “Magna Carta is a very important historical document but it has little significance for younger generations.”

Its significance will diminish further from September 2008, when it will be removed from the school curriculum. McGovern, who is also director of the History Curriculum Association, is not best pleased. “Magna Carta is seen as a dry topic with no place in a curriculum that puts the emphasis on lifestyle and citizenship,” he said.

Furthermore, it would be unlikely even to make the shortlist for Brown’s Britain Day. It is too “English”, sealed long before the formation of the United Kingdom.

So – even if the themes enshrined in Magna Carta were, as the historian Thomas Macaulay wrote, “engraven on the hearts of Englishmen” – it is likely to be an American institution forking out for the copy up for auction.

Redden expects that whoever buys it might return it to the National Archives in Washington, where the Perot foundation previously exhibited it, or to a similar establishment.

Americans perhaps revere Magna Carta more actively than the British do. Three times in the past four decades the American Bar Association has gathered at Runnymede to pledge adherence to its principles as the foundation of the rule of law.

Having referred to Magna Carta several times during his recent speech on liberty, Brown signed off with a stirring sentiment: “It is the challenge and the opportunity for our generation to write the next chapter of British liberty in a way that honours the progress of the past – and promises a more secure freedom to our children.”

Now Brown has launched his campaign to make over the constitution for the information age, we wait to see if the content will further protect or restrict freedom – in the famous words of Magna Carta – “for us and our heirs”.

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