One of the unintended consequences of the war on terror has been to reopen a question that has lain dormant for two centuries or more: whether the unwritten British Constitution should, like its American cousin, finally be written down. Just as the Americans once took constitutional lessons from the British, now the British are learning from the American Constitution.

This week, David Cameron, the Conservative leader, has made a radical proposal to address the way in which terrorist suspects use existing human rights law to avoid arrest or deportation.

Britain, Mr. Cameron says, needs its own Bill of Rights. Such a document would “guide the judiciary and the Government in applying human rights law when the lack of responsibility of some individuals threatens the rights of others.”

Mr. Cameron has hitherto traveled lightly when it comes to policy – he rides a bicycle to Westminster (with a limousine following behind, carrying his papers) – but a Bill of Rights is just about the weightiest Pandora’s Box imaginable. It makes President Bush’s task in enacting his proposed amendment to enshrine marriage in the U.S. Constitution look like a mere bagatelle.

Why would a British Bill of Rights be such a big deal? Because one of the few explicit principles governing the unwritten British constitution is that no Parliament may bind its successor. Mr. Cameron wants the next Parliament to do exactly that. He doesn’t yet know how – but he has a solution to that, too: appoint a “panel of distinguished jurists and other experts.”

I suspect that his panel will tell him that there is no way of entrenching a Bill of Rights in perpetuity, or even of accomplishing his more modest aim of excluding it from the Parliament Act, which allows the House of Commons to overrule the House of Lords in certain circumstances. As Mr. Cameron’s Oxford tutor, Professor Vernon Bogdanor, will have taught him, Parliament cannot renounce its sovereignty, even if it wanted to do so. Besides, the House of Commons is no more likely to hand over power to unelected judges than turkeys are to vote for Christmas.

Many have pointed out that Britain does, in fact, already have a Bill of Rights. It was passed in 1689 and set the seal on the Glorious Revolution, which overthrew King James II, a Catholic, and invited his Protestant daughter, Mary, and her Dutch husband, William, to occupy the throne.

The Bill of Rights was supposed to mark a pivotal moment, not only in the history of England, but in the history of the world: the end of the absolutist pretensions of the Stuart dynasty and the dawn of parliamentary democracy. It listed ancient rights and liberties, going back to Magna Carta, declared that laws made or suspended without Parliament were illegal and articulated the principle of “no taxation without representation,” that would later become the catalyst for the Declaration of Independence. The American Revolution was in many ways a Glorious Revolution writ large.

The Americans who drew up their own Bill of Rights a century later certainly had this precedent in mind, even though the Third Amendment – “Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof” – directly contradicts the main purpose of the British Bill: to establish the Protestant religion in its Episcopalian form, to the exclusion of all others.

Historians today are by no means agreed that this original 1689 Bill of Rights really did what was later claimed for it by the Whig theory of history: namely, to replace the divine right of kings with a human contract. Both William III and the Hanoverians still saw their kingship as ordained by God. Ironically, it was George III, hated by the American colonists as a tyrant, who first accepted the need for monarchy to be constitutional, embracing aristocracy and democracy in a new consensus.

Many believe that the delicate balance of the British constitution has been upset by the “pooling” of sovereignty in the European Union. But what has brought matters to a head has been, as so often in history, the need to defend the realm against a new threat: Islamist terrorism.

The European Court of Human Rights in Strasbourg is distinct from the E.U., but was created in 1953 to enforce a European Convention on Human Rights drawn up by the British. It can, and frequently does, overrule the British government.

The Blair government dealt with this by incorporating the European Convention into British law: the 1998 Human Rights Act. But this law, as interpreted by liberal judges, has severely hampered the war on terror. For example, in 2004 the law lords – Britain’s supreme court – ruled that suspected foreign terrorists could not be detained without trial. Nor could they be deported to their countries of origin, because the government is not allowed to balance the danger a terrorist suspect poses to public safety against the risk that a person may be tortured. Human rights law now trumps national security.

It is this problem that Mr. Cameron’s Bill of Rights is designed to address. Unlike the Human Rights Act, its provisions would reflect British circumstances. He has no intention of withdrawing from the European Convention, but the Strasbourg court would give more weight to a Bill of Rights, he claims, just as it does to other constitutional documents such as the German Basic Law.

There is precious little evidence for this. Even if all the domestic obstacles to a Bill of Rights were overcome, Mr. Cameron’s bid to free Britain from the human rights straightjacket would very likely remain a pious hope. The more carefully one examines it, the more this British Bill of Rights dissolves under scrutiny.

So why has he espoused such a double-edged doctrine? Is the Tory leader simply too young, inexperienced or naive? Or is he playing politics with the constitution? One senior Conservative I spoke to yesterday was cynical: “Just another gimmick,” he sighed wearily.

Another, more positive interpretation is possible, though. Mr. Cameron says he wants his Bill of Rights to “define the core values that give us our identity as a free nation.” So the purpose is as much symbolic as practical. It is all about forging a self-confident new patriotism, in the absence of which Britain looks vulnerable to an Islamist reverse-takeover.

Thus far, Mr. Cameron has said as little as possible about the war on terror, leaving many Conservatives anxious, even though he is running ahead of Tony Blair in the polls. Britons must hope that this constitutional initiative, problematic as it is, marks a new realization that Islamism is a threat, not just to peace and prosperity, but to our entire way of life. The British have entangled themselves in a human rights knot of Gordian proportions. A Bill of Rights wouldn’t cut that knot, but it might just help to rally a demoralized nation.