Marc Nadon cannot take his seat on the Supreme Court of Canada, the court's justices said Friday, in an unprecedented move that blocks Prime Minister Stephen Harper's latest appointment to the top court.

In a six-to-one decision, a majority of justices on the top court ruled that Nadon doesn't qualify to join them on the court.

The court ruled that its composition is constitutionally protected, and Parliament's attempt to change the Supreme Court Act through a budget bill is unconstitutional.

The government introduced changes in last December’s budget bill in an effort to make Nadon eligible as a former member of the Quebec bar — as opposed to a current member. The court says that section is void because the government does not have the power to make such amendments unilaterally.

Changes to the court's makeup require a constitutional amendment with the unanimous consent of the provinces, the court says.

Nadon's appointment to the country's top court is declared void retroactively, and he is deemed to have never been sworn in.

A spokesman for Prime Minister Stephen Harper said Friday afternoon the government is "genuinely surprised" by the decision.

"Prior to Justice Nadon's appointment, the Department of Justice received legal advice from a former Supreme Court justice, which was reviewed and supported by another former Supreme Court justice, as well as a leading constitutional scholar. None of them saw any merit in the position taken by the court," Stephen Lecce said in a statement.

"We will review the details of the decision and our options going forward."

Implications for Senate reference

University of Ottawa lawyer Sebastien Grandmond, centre, talks to reporters in the lobby of the Supreme Court Friday. (Fred Chartrand/Canadian Press) "Today's judgment will be of great importance, especially in constitutional matters," said Sébastien Grammond, who represented two groups of retired Quebec judges in the case. "First and foremost because it makes important statements as to how the Constitution of Canada can be amended."

The decision may also provide insight into how the court will rule on the Senate reference, Grammond suggested, which includes the question of whether Parliament can reform the Senate without reopening the Constitution.

"It will take months and years perhaps to understand" all the implications of the decision, he said.

Toronto lawyer Rocco Galati had challenged Harper's appointment of Nadon. Galati argued he didn't qualify for one of the court's three Quebec seats, because he came from the Federal Court and not from a Quebec court.

Quebec judges are a special category because of the province's unique civil legal code, different from the common law applied in the rest of the country.

Harper referred the question to the court and Nadon stepped aside until the matter could be decided.

Nadon has been collecting a salary since he was named. It's now up to the registrar of the Supreme Court to decide whether he has to pay the money back, likely the first time the court has ever faced this question. Nadon's annual salary would have been $351,700. He was named in October, so has been receiving a pay cheque for about six months.

One option the registrar is considering is having Nadon repay the difference between the Supreme Court and Federal Court salaries.

Court silent on some questions

In its ruling, the court repudiated the government’s claim that Nadon was eligible because the Constitution does not actually say he has to be a “current” member of the Quebec bar.

“In our view,” the ruling says, “the answer to this question is no: a current judge of the Federal Court of Appeal is not eligible for appointment under [section 6] as a person who may be appointed 'from among the advocates of that province.' This language requires that, at the time of appointment, the appointee be a current member of the Quebec bar with at least 10 years standing.”

I feel for Justice Nadon this morning. He in no way is responsible for anything that is happening right now. - Françoise Boivin, NDP justice critic

The court is silent on whether Nadon could be appointed from another province, or whether he could simply re-join the Quebec bar, where he began his career, and then be properly named.

Justice Michael Moldaver was the only dissenter from today’s ruling.

Justice Marshall Rothstein, also appointed from the Federal Court, recused himself. He has given no public reasons for that, but it removes the perception of a conflict of interest.

Rothstein is from Manitoba and remains on the Supreme Court because the rules governing Quebec appointments are different. They impose additional requirements, the Supreme Court says, which Nadon does not meet.

Three of the six justices who ruled against Nadon's appointment were appointed by Harper, as was Moldaver. The remaining three justices who ruled against the appointment were appointed by previous governments.

Quebec missing judge

Opposition Justice Critic Francoise Boivin called on Justice Minister Peter MacKay to move quickly to fill the Quebec vacancy left open by the rejection of Marc Nadon. (Fred Chartrand/Canadian Press) New Democrat justice critic Françoise Boivin said it has been a year now that Quebec has gone without one of its judges on the Supreme Court.

She said Justice Minister Peter MacKay should follow the existing process: talk to lawyers in Quebec and consult the chief justice of Quebec's Superior Court, the Quebec bar association and the Canadian Bar Association.

"It's hard to play politics with that decision," she said. "We shouldn't play politics, in my book, with justice issues like this."

"I feel for Justice Nadon this morning. He in no way is responsible for anything that is happening right now."

Boivin was on the committee of MPs that advised on a final list of candidates from which Harper chose Nadon. She said it wasn't a unanimous decision.

Mobile users, read the Supreme Court's ruling here.