He had argued that his arrest pending an extradition request by a Spanish judge investigating allegations of murder, torture and kidnapping was unlawful.

Quashing the arrest warrants, Lord Bingham, the Lord Chief Justice, said: "The applicant is entitled as a former head of state to immunity from civil and criminal proceedings of the English court."

But he said the 82-year-old general should remain in detention "pending the termination of any appeal against this decision".

Joshua Rozenberg: "Judgement is the very best Pinochet could have hoped for"

He was also awarded legal costs, believed to be about £350,000.

The Crown Prosecution Service announced afterwards it would appeal against the ruling.

Papers for the appeal are to be submitted by 1600 GMT on Monday, meaning the case could be heard in the House of Lords, the most senior court in the UK, by the end of next week.

Gen Pinochet's lawyers said they would apply for his release on bail at Bow Street Magistrates Court.

Magistrate Graham Parkinson is considering sitting outside the court, and hearing the details at the ailing general's bedside, perhaps as early as Friday.

Chile hails ruling

Chile welcomed the High Court ruling. Its Deputy Foreign Minister, Mariano Fernandez, told reporters in London: "The Chilean government is very happy and satisfied that the British High Court has recognised Senator Pinochet's immunity."

But the judge who ordered his detention, Judge Baltasar Garzon, has promised to continue to seek his extradition to Spain.

There were angry scenes outside the court as police held back demonstrators who chased officials down the street.

About a dozen protesters chanted abuse as officials walked through police lines.

Jane Peel reports from the High Court

Gen Pinochet was detained in London on 16 October while recuperating from surgery.

He was arrested at the request of Spanish judges seeking to extradite the former dictator to face charges related to more than 4,000 political killings alleged to have taken place during his 1973-1990 rule in Chile.

Speaking for the general, Clive Nicholls, QC, had argued that his arrest was not legally justified on several grounds, including immunity from arrest for actions committed while he was head of state.

He told the court on Monday that both the original warrant and an "unprecedented" second warrant were "fatally flawed" and that a United Nations court would be the proper forum.

Other requests pending

Since the arrest, Switzerland has also applied for his extradition and Sweden could follow suit.

But the BBC's legal affairs correspondent, Joshua Rozenberg, says the High Court decision looks likely to rule out all other extradition requests.

General Pinochet: Will not be tried under UK law

He said the judge ruled that only an international tribunal could have dealt with the charges outside Chile because of the law of "sovereign immunity", which protects heads of state while in other countries.

In a separate development, the Attorney General, John Morris, QC, has refused to give his consent for the private prosecution of Gen Pinochet on torture offences by anyone in the UK.

He was responding to a case brought to him by four alleged victims of his regime.

Joshua Rozenberg on the implications of the judgement

Lawyers acting for three Chileans living in the UK and a fourth based in Lebanon had requested permission to pursue a private criminal prosecution under the Criminal Justice Act 1988.

Mr Morris's office said the applications for consent were turned down because "there is insufficient admissible evidence under English law of an offence".

Blair defends handling

Meanwhile the Prime Minister, Tony Blair, dismissed a Tory attack over the detention of Gen Pinochet.

Mr Blair reiterated that the government was not involved in the arrest.

He said: "The judicial process has not involved the government issuing warrants for arrest.