THE TIME HAS COME TO TALK of English liberty — of Magna Carta and the abolition of Star Chamber, of Milton and free speech. When politicians threaten to detain suspects without trial and to jail radical preachers and to put pressure on judges, it is worth recalling how their forebears fought against these very abuses of power. Liberty does not date from the adoption of the European Convention on Human Rights. On the contrary, it was forged in the crucible of a civil war that cost one Englishman in ten his life, culminating in the trial for tyranny of a head of state who intimidated judges and refused political or religious freedom.

English historians have been unnecessarily queasy about the trial and execution of Charles I, the momentous event that ended absolutism and can now be seen as the precursor of proceedings against rulers such as Pinochet, Milosevic and Saddam who make war on their own people. Although we prefer to teach our children history through the indulged lives of kings and queens, it was the republicans who achieved the rights that Western democracies now insist are universal. These Puritans, with their fused passions for liberty and God, first established representative government. They understood — rather more clearly than Charles Clarke or Hazel Blears — that the precondition of parliamentary sovereignty is that all legislation must conform with rights declared and enforced by judges who are free from political pressure.

Magna Carta was reinvented in 1628 as the Petition of Right — Parliament’s first attempt to limit Stuart absolutism. Charles I ignored it and his lickspittle judges, who held office “at the King’s pleasure”, defied habeas corpus by leaving Sir John Eliot MP to die in the Tower. The Star Chamber — an arm of the executive — jailed Presbyterian preachers for sedition. In the ship money case, a majority of the judges decided that a tax could be imposed without parliamentary approval. “Rex is lex,” they ruled: the king was above the law and could dispense with fundamental rights whenever, in his subjective judgment, national security demanded.

These were some of the events that provoked the Civil War, fought from 1642 to 1648, and still enthusiastically re-enacted in muddy fields every summer. Parliament reversed the decision, abolished the Star Chamber and determined that judges should hold office for life, subject only to good behaviour. But these great constitutional reforms were in jeopardy so long as Charles I remained head of state, actively planning a third war from his confinement at Carisbrooke Castle. So the army leaders and the independent group of MPs moved against the appeasers in Parliament and brought the King to trial.

In January 1649, lawyers fled the Inns of Court to avoid involvement in the prosecution. The brief was accepted by a barrister named John Cooke: it instructed him to frame a charge “to the end that no chief officer may hereafter presume to enslave or destroy the English nation and expect impunity for so doing”.

This use of the word impunity enters history in Cooke's brief. He deposed more than 30 witnesses to prove Charles’s command responsibility — not merely for starting the conflict but also for supervising the torture of prisoners of war and for directing plunder and pillage of the homes of non-combatants. At the opening of the trial, held in a packed Westminster Hall, the King hit the prosecutor with his cane: its silver tip was dislodged and rolled between the two men. Cooke’ s refusal of the King’s command to retrieve it led to that iconic moment when Charles was obliged to bend to pick it up himself — a moment that gave meaning to what had previously been an empty aphorism: Be you ever so high, the law is above you. Historians have caricatured it as a “show trial” although it was uniquely fair by the standards of the time and its verdict was far from pre-ordained — it was crucially influenced by the King’s lack of remorse for deaths on both sides during the Civil War. His execution brought about a short-lived republic but, at the Restoration, Cooke and the surviving judges were subjected to a rigged trial at the Old Bailey, followed by disembowelling at Charing Cross in the presence of Charles II.

Not even the rabid royalist resurgence in 1660 could bring back the Star Chamber, although habeas corpus was cleverly circumvented. The King’s judges died bravely and aroused such public sympathy that the Government feared to bring more republicans to trial: the Lord Chancellor directed that they should remain imprisoned on offshore islands such as Jersey, where the “great writ” could not reach. This device — the Bush administration’s precedent for Guantanamo Bay — was reversed by the Habeas Corpus Act in 1679, a statute applied by the US Supreme Court in 2004 when they ordered that detainees should have due process.

The principles of those who established parliamentary sovereignty cannot be brushed aside. They insisted that there could be no secret trials, no months of detention at executive discretion, no nobbling of judges. The Prime Minister may say that, after the London bombings “the rules have changed”, but some rules are immutable. Parliament can derogate from the European Convention, but cannot alter its own constitutional history.

Geoffrey Robertson, QC, is a UN appeal judge. He talks at The Times Cheltenham Literary Festival tomorrow at 10am. Tickets £6 from 01242 227979

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