She pointed to autocracies in the developing world and former Communist countries as lessons on where interference with the judiciary might lead. "It takes a lot of degeneration before a country falls into dictatorship, but we should avoid these ends by avoiding these beginnings."

In her address to an audience of corporate lawyers on Thursday, Ms O'Connor singled out a warning to the judiciary issued last year by Tom DeLay, the former Republican leader in the House of Representatives, over a court ruling in a controversial "right to die" case.

After the decision last March that ordered a brain-dead woman in Florida, Terri Schiavo, removed from life support, Mr DeLay said: "The time will come for the men responsible for this to answer for their behaviour."

Mr DeLay later called for the impeachment of judges involved in the Schiavo case, and called for more scrutiny of "an arrogant, out-of-control, unaccountable judiciary that thumbed their nose at Congress and the president".

Such threats, Ms O'Connor said, "pose a direct threat to our constitutional freedom", and she told the lawyers in her audience: "I want you to tune your ears to these attacks ... You have an obligation to speak up.

"Statutes and constitutions do not protect judicial independence - people do," the retired supreme court justice said.

She noted death threats against judges were on the rise and added that the situation was not helped by a senior senator's suggestion that there might be a connection between the violence against judges and the decisions they make.

The senator she was referring to was John Cornyn, a Bush loyalist from Texas, who made his remarks last April, soon after a judge was shot dead in an Atlanta courtroom and the family of a federal judge was murdered in Illinois.

Senator Cornyn said: "I don't know if there is a cause and effect connection, but we have seen some recent episodes of courthouse violence in this country ... And I wonder whether there may be some connection between the perception in some quarters, on some occasions, where judges are making political decisions yet are unaccountable to the public, that it builds up and builds up to the point where some people engage in violence."

Although appointed by a Republican, Ms O'Connor voted with the supreme court's liberals on some divisive issues, including abortion, making her a frequent target for criticism from the right. After announcing that she intended to retire last year at the age of 75, she was replaced in February this year by Samuel Alito, who is generally regarded as being more consistently conservative.

In her speech, Ms O'Connor said that if the courts did not occasionally make politicians mad they would not be doing their jobs, and their effectiveness "is premised on the notion that we won't be subject to retaliation for our judicial acts".