Prime Minister Stephen Harper says he's surprised to learn that Quebec has different rules when it comes to appointing judges to the Supreme Court of Canada.

Justice Minister Peter MacKay, in explaining his boss' reaction, described the regulations as "new."

The rules have been in effect since 1875, although MacKay denies that and says it was an interpretation by the Supreme Court.

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Harper was responding to questions in the House of Commons about his government's recent fumbled attempt to name Marc Nadon to the high court.

The Supreme Court itself ruled recently that Nadon, a Federal Court judge, did not fit the criteria to be named to the high court.

Harper has said he will follow the letter and the spirit of the ruling.

MacKay says the decision sets new rules for the nomination of Quebec judges to the Supreme Court.

"It limits the pool of judges who can go to the Federal Court now from Quebec if they have any aspirations of going to the Supreme Court of Canada," he said Tuesday outside the Commons.

"And it limits the pool of eligible judges from the province of Quebec for the Supreme Court of Canada, which I think is unfortunate. It creates a double standard."

Opposition Leader Tom Mulcair said the situation could have been easily resolved if the government had done its homework.

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"It's enough to read the law on the Supreme Court to understand that the nomination of Judge Nadon did not conform," the New Democratic Party leader said.

Mulcair said that even though the prime minister had recognized the Quebecois as a nation in the House of Commons he has since refused to offer any concrete effects.

Liberal Leader Justin Trudeau said Harper has to take responsibility for having a Supreme Court seat vacant for several months and said the prime minister showed a lack of judgment in his choice of a candidate to fill it.

MacKay would not say when another judge will be nominated to fill the vacant spot.