OTTAWA -- Prime Minister Stephen Harper says he ignored a legal caution from Canada's chief justice on the eligibility of a potential Supreme Court nominee last summer because to listen to such advice would have been improper.

In an unprecedented prime ministerial rebuke of the top court, Harper unloaded Friday on Chief Justice Beverley McLachlin, one day after government leaks and an extraordinary exchange of public statements revealed a deep rift between the two.

Speaking to reporters in London, Ont., Harper said McLachlin acted improperly last July in advising his office that Marc Nadon, a Federal Court of Appeal judge, might not fit the legal criteria set for Quebec justices under the Supreme Court Act.

At the time, Nadon was one of three judges on a short list of potential appointees being vetted by a panel of MPs. That panel had sought McLachlin's advice.

"I can tell you this," said a clearly irked Harper, who appointed Nadon last September after commissioning independent legal advice that approved of the choice.

"I think if people thought that the prime minister, other ministers of the government, were consulting judges before them or -- even worse -- consulting judges on cases that might come before them, before the judges themselves had the opportunity to hear the appropriate evidence, I think the entire opposition, entire media and entire legal community would be outraged," he said.

"So I do not think that's the appropriate way to go."

McLachlin's office responded with its own public statement, the second in as many days.

McLachlin spoke to Harper "as a courtesy" last April to give him the retirement letter of justice Morris Fish, said the statement.

She met with the parliamentary vetting committee on July 29 "as part of the usual process," then contacted Justice Minister Peter MacKay and the Prime Minister's Office on July 31 "to flag a potential issue regarding the eligibility of a judge of the federal courts to fill a Quebec seat on the Supreme Court."

McLachlin's office said it contacted the PMO to make "preliminary inquiries" about setting up a call or meeting with Harper on the matter, "but ultimately the chief justice decided not to pursue a call or meeting."

"Given the potential impact on the court, I wished to ensure that the government was aware of the eligibility issue," McLachlin said in her statement.

"At no time did I express any opinion as to the merits of the eligibility issue. It is customary for chief justices to be consulted during the appointment process and there is nothing inappropriate in raising a potential issue affecting a future appointment."

Harper's office, meanwhile, issued a statement late Thursday saying the prime minister was advised by his justice minister that accepting a call from McLachlin would be "inadvisable and inappropriate."

"The prime minister agreed and did not take her call," said the statement from communications director Jason MacDonald.

Harper ended up nominating Nadon, a semi-retired 64-year-old with a specialty in maritime law, last Sept. 30 and he was duly sworn in to the top court by McLachlin on Oct. 3.

However, constitutional lawyer Rocco Galati challenged the appointment, and in March this year the Supreme Court ruled 6-1 that Nadon did not meet the legislated eligibility requirements.

Court watchers were agog Friday at the spectacle.

Emmett Macfarlane, an assistant professor at the University of Waterloo who has written a critical history of the Supreme Court entitled "Governing From the Bench," called the public dispute unprecedented in Canadian history.

Macfarlane said court experts may debate whether McLachlin was imprudent in contacting the Prime Minister's Office about her concerns.

"It was perhaps unwise, that's all I'll say," he said. "But for the prime minister to engage in this kind of spitting match with the court and with the chief justice is absurd."

Macfarlane called the public breach of confidentiality "mind boggling."

"Other than if they see a fight with the court like this itself as some sort of political gain, it's unfathomable to me what exactly the benefit here is."

Mel Cappe, a former clerk of the Privy Council, the top federal bureaucrat, said one of McLachlin's key roles as chief justice is to safeguard the integrity of the court.

"The chief justice and the prime minister have an interest in maintaining the legitimacy of the Supreme Court," Cappe said.

"If the chief were preoccupied with the appointment process, it would be only in so far as it maintained the legitimacy and respect for the bench."

Opposition critics said Harper's decision to take his concerns public undermines the court.

"It's so cheap that ... it's beyond contempt," said NDP justice critic Francoise Boivin, who sat on the vetting committee that examined Nadon and other unnamed candidates.

"I wish the chief justice had the power to cite for contempt what the minister of justice and the prime minister of Canada (are) doing right now."

Harper said Friday he consulted constitutional and legal experts both within and outside the government, and they argued Nadon was eligible.

And he said the Supreme Court decision to reject Nadon means that Federal Court judges from Quebec are essentially ineligible to sit on the high court, a situation he considers unfair.

"The reality is the Supreme Court has decided that a Quebec judge at the Federal Court is a second class judge," said Harper.

He said Quebec jurists may no longer aspire to the Federal Court because it will rule them out of a shot at the Supreme Court.

However, Federal Court judges from Quebec remain eligible to sit on the top bench -- and in fact at least one has in the past -- but not in one of the three seats explicitly reserved for Quebec jurists.

- With files from Jennifer Ditchburn.