Ross Perot, the Texan billionaire and former presidential candidate, is selling his Magna Carta next Tuesday in New York. Perot's copy of the 13th-century manuscript, which he purchased from an English family 23 years ago, is the only one in private hands, and one of only two outside Britain (the other belongs to the Australian Government).

The remaining 15 copies of Magna Carta are all held by British museums and other institutions, so next week's auction is the first, and almost certainly the last, time that this, the most important legal document in the world, will go under the hammer.

With a bit of luck, and a lot of money (perhaps $30 million), it may stay in the US. I hope so, because America badly needs it.

Magna Carta, the great charter, the very birth certificate of liberty, is more widely hallowed today than ever before, yet the principles it upholds are in serious danger, undermined by the demands of the “War on Terror”: by CIA torture of terrorist suspects, by control orders in this country and efforts to extend the detention of suspects without charge, by “extraordinary rendition” and above all by the shackled and shuffling ghosts of Guantanamo Bay.

When the grumpy barons forced King John to put his seal on Magna Carta in a field near Staines in 1215, they were acting out of pure feudal self-interest. They did not intend to forge a document enshrining the rights of the individual against the State; but that is what Magna Carta evolved into, and remains.

Today perhaps one third of the world's population is governed in accordance with the broad principles laid down in an English meadow eight centuries ago: that no person is above the law, and no person may be persecuted by power. The most enduring legacy of Magna Carta is, of course, the writ of habeas corpus, the most fundamental defence against unlawful executive detention and arbitrary rule.

The great charter states in Clause 39: “No free man shall be taken or imprisoned or disseised or exiled or in any way destroyed, nor will we go upon him nor will we send upon him except upon the lawful judgement of his peers or the law of the land.” In this country, Magna Carta has attained a semi-mythical status, a sacrosanct document deployed to bolster a bewildering array of causes and politics.

In a recent speech on liberty, Gordon Brown made no less than four allusions to the charter. When the French were celebrating the anniversary of their revolution in 1989, Margaret Thatcher was heard to point out to François Mitterrand: “We, of course, had the Magna Carta.” The charter was used to rally unhappy subjects against despotic power, both to demand and to prevent political change; it was recruited in the English Civil War as a weapon against the royal prerogative, and by 19th-century Chartists calling for parliamentary change and a people's charter. The radical John Wilkes considered it “the distinguishing characteristic of all Englishmen”, while the historian Thomas Macaulay believed the charter's principles were engraved on every English heart.

Oddly, he may still be right: in a recent poll by the BBC to find a day on which to celebrate “Britishness”, June 15, the day that the charter was signed, emerged as the popular and unexpected winner.

But if Magna Carta is still respected here, it is even more venerated in the US. In the years before the American Revolution, colonial lawyers repeatedly invoked its principles against the Government in London. Alexander Hamilton considered habeas corpus to be the “bulwark” of individual liberty, and condemned secret imprisonment as the most “dangerous engine of arbitrary government”.

The framers of the Constitution were specific: habeas corpus could never be suspended, “unless when in cases of rebellion or invasion the public safety may require it”. The Fifth Amendment simply rephrased Clause 39: “No person shall... be deprived of life, liberty, or property, without due process.” The American Bar Association periodically gathers at Runnymede to rededicate itself to the ideas first established there. The charter even made an appearance in the Paula Jones sexual harassment case in 1994, when the judge refused to delay proceedings until after Bill Clinton left office, pointing out that “it is contrary to our form of government, which asserts, as did the English in the Magna Carta... that even the sovereign is subject to God and the law”.

That reverence is worth recalling today, when the White House is accused of authorising the “waterboarding” of detainees (or “enhanced interrogation”, to give it the correct, revolting euphemism), when hours of videotaped interrogations have been destroyed, when hundreds are still detained without charge in Guantanamo, including some already cleared for release. Magna Carta states: “To none will we deny or delay right or justice,” yet the Government in Britain, having failed to extend detention for terrorist suspects to 90 days, is now pushing for a 42-day extension.

Times of crisis and war, when national security is under direct threat, may justify the temporary suspension of individual liberties, but in the six years since 9/11 we have seen a steady erosion of the principles of the Magna Carta, with the seizure, imprisonment, exile and delay outlawed by that great document increasingly deployed as accepted instruments of foreign policy.

For the last 20 years, Perot's Magna Carta has been displayed in a gold-plated case in Washington, alongside the Declaration of Independence and the US Constitution, its direct descendants. For the sake of the future, as well as the past, America should buy Magna Carta next week, put it back on display and read it again.

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