Prime Minister Stephen Harper has accused Supreme Court Chief Justice Beverley McLachlin of breaching a basic rule of her office, as a deepening conflict between the government and the country's highest court breaks out into a public dispute.

The Prime Minister's Office publicly asserted that the Chief Justice attempted to contact Mr. Harper about a court case, and said that he refused to take her phone call when Justice Minister Peter MacKay told him it would be "inappropriate."

The case involved Mr. Harper's Supreme Court appointment of Justice Marc Nadon, whom the court eventually ruled ineligible. Both the public dispute between a prime minister and a chief justice, and the allegation itself, are unprecedented.

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"Neither the Prime Minister nor the Minister of Justice would ever call a sitting judge on a matter that is or may be before their court," the Prime Minister's Office said in a statement released early Thursday evening.

"The Chief Justice initiated the call to the Minister of Justice. After the Minister received her call he advised the Prime Minister that given the subject she wished to raise, taking a phone call from the Chief Justice would be inadvisable and inappropriate. The Prime Minister agreed and did not take her call."

It does not specify when the alleged attempt at contact occurred.

Any attempt at contact about a case would be a serious breach of the separation between the judicial and executive branches of government, without a known precedent in Canadian history, according to Supreme Court historian Frederick Vaughan.

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"It's absolutely unheard of that a judge would call a member of cabinet or government in a case that is before the court. It's an inflexible rule," he said, adding that he thinks Chief Justice McLachlin would have to resign if she broke that rule.

Chief Justice McLachlin, the country's longest-serving chief justice, who has been at the head of the court since 2000, was giving a speech Thursday night on women and the law at the University of Moncton, and a spokesman for the court said she was not available to respond.

But in a statement issued a day earlier, in response to a reporter's questions, her spokesman, Owen Rees, said she had been consulted by the parliamentary screening committee on the government's short list of candidates before the appointment was made, and commented on the needs of the court. She had also raised the question of a Federal Court judge's eligibility with Mr. MacKay and the Prime Minister's chief of staff, Ray Novak, he said.

Justice Nadon was (and remains) a judge on the Federal Court of Appeal. No Federal Court judge had ever filled one of the three spots on the court reserved for Quebec judges. The Supreme Court Act did not specify expressly that Federal Court judges were eligible.

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"The question concerning the eligibility of a federal court judge for appointment to the Supreme Court under the Supreme Court Act was well-known within judicial and legal circles," the statement from Mr. Rees said. "Because of the institutional impact on the Court, the Chief Justice advised the Minister of Justice, Mr. MacKay, of the potential issue before the government named its candidate for appointment to the Court. Her office had also advised the Prime Minister's Chief of Staff, Mr. Novak. The Chief Justice did not express any views on the merits of the issue."

The Prime Minister's statement said it was issued in response to her statement.

Mr. Vaughan, author of The Supreme Court, a history of the court published in 1984, said he flatly does not believe it. "I don't believe for a moment the Chief Justice would call the Prime Minister about the appointment of a judge. And especially Beverley McLachlin. This is pretty shoddy stuff. All it does is cast aspersions on the Chief Justice."

The government has lost five major cases at the Supreme Court in the past six weeks, with only one voice of support from the court's eight sitting members, just once.

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One Conservative MP said the government wants it made clear it didn't consult the Chief Justice on the Supreme Court appointment. The Tories feel there should be a firm wall between the bench and the decisions they make as to who should sit on it.