OTTAWA -- The Supreme Court of Canada has rejected Prime Minister Stephen Harper's latest appointee to its ranks by a 6-1 margin in a stinging rebuke of the Conservative government.

The court said that Marc Nadon, the semi-retired Federal Court of Appeal judge from Quebec, is not eligible to sit on the court because he does not meet the specific requirements for the jurists appointed from that province.

The high court also struck down a key feature of the Conservative bill that last year amended the Supreme Court Act.

The court said Parliament cannot unilaterally change the composition of the Supreme Court because its essential features are constitutionally protected under the Constitution Act.

Nadon was declared ineligible because he came from the Federal Court and did not meet the criteria of either coming from the Quebec Superior Court, the Quebec Court of Appeal, or being a current member of the Quebec bar.

The court's opinion affects only Quebec appointments, which means there is no impact on current Supreme Court Justice Marshall Rothstein of Manitoba, whom Harper appointed from the Federal Court of Appeal in 2006.

The Tories amended the Supreme Court Act last year to allow the appointment of a jurist who is currently or has in the past been a member of the Quebec bar with at least 10 years standing -- a provision that applies to Nadon. At the same time, Harper referred the appointment to the Supreme Court for an opinion.

"The narrow question is thus whether he was eligible for appointment because he had previously been a member of the Quebec bar," said the ruling, which -- in a rare occurrence -- was formally written by all six judges in the majority.

"In our view, the answer to this question is no: a current judge of the Federal Court of Appeal is not eligible for appointment."

The only dissenter was Justice Michael Moldaver, a Harper appointee from Ontario.

The high court said the government did not have the authority to amend the Act, saying "the unanimous consent of Parliament and all provincial legislatures is required for amendments to the Constitution relating to the 'composition of the Supreme Court'."

Harper's decision in September to name the 64-year-old Nadon has been controversial from the outset.

The court said Nadon's appointment was void and that he never actually became a member of the court. Nadon retains his status as a supernumerary Federal Court judge.

The court did not address the hypothetical question of whether Nadon could resign as a judge and go back to Quebec and reactivate his status as a lawyer to meet the eligibility requirements.

"We note in passing that the reference questions do not ask whether a judge of the Federal Court or Federal Court of Appeal who was a former advocate of at least 10 years standing at the Quebec bar could rejoin the Quebec bar for a day in order to be eligible for appointment to this Court under Sec. 6," the court said Friday.

"We therefore do not decide this issue."

The court's decision comes as Harper departs Canada for a week-long European trip that will take him to Ukraine on Saturday.

Harper's decision to name Nadon to one of three Supreme Court seats reserved for Quebec was challenged by Toronto lawyer Rocco Galati, which triggered an unprecedented chain of events.

Friday's opinion represents a serious political rebuke to Harper and will have implications for the next Quebec vacancy that opens up later this year. Justice Louis LeBel reaches the mandatory retirement age in November.

The case could resonate on the Quebec provincial election trail.

Nadon had been strictly quarantined from the eight remaining justices and is staying away from country's highest courthouse. Rothstein, too, recused himself from the case, but gave no public reasons for doing so.

Rothstein's Manitoba credentials were based on a legal career in the province from 1966 to 1992 when he was a partner in a law firm and a lecturer in transportation law at the University of Manitoba. He was appointed to the Federal Court trial division in 1992.

In all, the court heard from seven interveners, including the federal and Quebec governments, as well as an association of provincial court judges and constitutional experts.

The political ripples will flow from a very specific set of legal issues that lie at the core of the case.

It all centres on two specific sections of the Supreme Court Act -- sections that were rewritten by the Conservative government and buried in its 300-page plus omnibus budget implementation bill last year.

Section 5 of the Supreme Court Act deals with the general eligibility of nominees. It allows for the appointment of a former or current member of the bar, a member of a provincial superior court, or a barrister with 10 years standing in the province. It is unaffected by the opinion.

But the court rejected Section 6, which deals specifically with the appointment of the three mandatory Quebec appointees. The court has also been asked to consider whether the government had the constitutional right to amend the Supreme Court Act.

Friday's opinion was unusually swift after the court heard arguments in January.

Galati argued there was no "constitutional imperative" to extend eligibility to "every single lawyer in the country" to be a candidate for the Supreme Court.

The federal government argued that interpreting the Supreme Court Act to bar any Federal Court judge from being appointed to the top court would reduce the already small pool of qualified judges.

The Supreme Court had no issue with reducing that pool.

"By specifying that three judges shall be selected from among the members of a specific list of institutions, Sec. 6 requires that persons appointed to the three Quebec seats must, in addition to meeting the general requirements of Sec. 5, be current members of these institutions," the court said Friday.