Habeas corpus (ad subjiciendum) is Latin for "you may have the body" (subject to examination). It is a writ which requires a person detained by the authorities be brought before a court of law so that the legality of the detention may be examined.

The name is taken from the opening words of the writ in medieval times.

Although rarely used nowadays, it can theoretically be demanded by anyone who believes they are unlawfully detained and it is issued by a judge.

It does not determine guilt or innocence, merely whether the person is legally imprisoned. It may also be writ against a private individual detaining another.

If the charge is considered to be valid, the person must submit to trial but if not, the person goes free.

The Habeas Corpus Act passed by Parliament in 1679 guaranteed this right in law, although its origins go back much further, probably to Anglo-Saxon times.

Habeas corpus has a mythical status in the country's psyche Michael Zander QC

Sir William Blackstone, who wrote his famous Commentaries on the Laws of England in the 18th Century, recorded the first use of habeas corpus in 1305. But other writs with the same effect were used in the 12th Century, so it appears to have preceded Magna Carta in 1215.

Its original use was more straightforward - a writ to bring a prisoner into court to testify in a pending trial. But what began as a weapon for the king and the courts became - as the political climate changed - protection for the individual against arbitrary detention by the state.

It is thought to have been common law by the time of Magna Carta, which says in Article 39: "No freeman 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."

Over the next few hundred years, concern grew that kings would whimsically intervene on matters of detention, so it was enshrined in law in 1679.

Night calls

In 1772, there was a landmark case in which it was invoked. James Somersett, a black slave brought back to the UK from Jamaica, was freed after a debate sparked by his demand for habeas corpus. Lord Mansfield successfully argued for his release.

These days it is rarely used, although it has greater effect in the US, where its most common use is by prisoners after conviction.

Michael Zander QC, Emeritus Professor of Law at the London School of Economics, says: "Habeas corpus has a mythical status in the country's psyche.

King John's clash with the barons strengthened the rights of habeas corpus

"In reality it is no longer of great practical significance as there are today very few habeas corpus applications, but it still represents the fundamental principle that unlawful detention can be challenged by immediate access to a judge - even by telephone in the middle of the night."

It no longer plays a role in regard to detention by the police as it has been superseded by the much more detailed and workable provisions of the Police and Criminal Evidence Act 1984, which lays down precise rules about the length of pre-charge detention, he adds.

But there have been occasions when the British Parliament has suspended it, usually in times of social unrest.

William Pitt, startled by the success of the French revolution, did so after France declared war on Britain in 1793, to arrest parliamentary reformers. This was repeated by Lord Liverpool's government against the same movement in 1817.

War was a particularly fraught time for individual liberty. The Defence of the Realm Act 1914 meant the home secretary could intern residents and it was used against people of German descent, and Irish suspected of involvement in the Easter Uprising.

These powers were reinstated in World War II to detain those of German background, including Jewish refugees, as well as those with known fascist sympathies, such as Oswald Mosley. At the same time, the US authorities interned more than 110,000 Japanese-Americans.

Everyone has a right to habeas corpus and that will remain the case Home Office spokesman

But the most recent example happened in 1971, when the British Government introduced the internment of hundreds of republican suspects in an attempt to shut down the IRA. The tactic was abandoned four years later and is thought to have increased support for the IRA.

Whether the anti-terror bill is the latest chapter in the history of habeas corpus is a matter of debate. Boris Johnson MP said earlier in the week that Tony Blair is the first peacetime prime minister to curtail the right to habeas corpus.

Since then, the Lords appears to have won a concession that all the control orders issued against terror suspects be made by judges, not by ministers. And Home Secretary Charles Clarke insists there is no plan to detain anyone under the new laws.

Mr Zander says: "The Anti-terrorism, Crime and Security Act 2001 passed in the aftermath of 9/11 set aside habeas corpus in regard to terrorism suspects who cannot be prosecuted. The Prevention of Terrorism Bill now before Parliament would broaden the ways in which terrorism suspects can be dealt with without being charged or prosecuted."

But the Home Office denies its plans amount to habeas corpus suspension. A spokesman said: "We are not removing habeas corpus rights. Everyone has a right to habeas corpus and that will remain the case."