The NDP is going to court to challenge the verdict of a secretive House of Commons committee, which found New Democrat MPs guilty of improperly spending $1.17 million in parliamentary resources on partisan mass mailings.

The multi-party board of internal economy ordered New Democrat MPs on Wednesday to repay $36,000 to the Commons — presumably to cover the cost of envelopes used to send almost 2 million missives to households in 26 ridings, including four in the midst of byelections.

And it concluded the MPs owe another $1.13 million to Canada Post for allegedly improper use of their free parliamentary mailing privileges, known as franking.

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"The board cannot compel members to repay these (franking) costs," Conservative whip and board spokesman John Duncan said late Wednesday.

"However, it is incumbent on these members to work with Canada Post to repay the $1.13 million."

Canada Post spokeswoman Anick Losier said in an email late Wednesday that the Crown corporation would review the board's decision and would have "no further comment at this time."

The board oversees spending in the Commons and polices how MPs spend their office budgets and use other parliamentary resources.

But New Democrats dismissed the board as a "kangaroo court" and its verdict as the foregone conclusion of a partisan "witch hunt." They vowed to take the matter to a real court for a fair and impartial hearing.

"None of this will hold up in a court of law," asserted NDP House leader Peter Julian, who was accompanied to a late news conference by renowned constitutional lawyer Julius Grey.

"We believe that we have followed the member's bylaws and the Canada Post Act, and we will now have to fight this process — by a partisan secretive body that is dominated by Conservatives — outside of Parliament."

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Julian said Conservatives, aided and abetted by the Liberals, acted as "judge, jury and executioner," nailing the NDP for allegedly partisan content in their mailings while sanctioning similar mailouts by their own members.

The NDP was not given a hearing, nor was the party allowed to give evidence or call witnesses on behalf of its MPs, he said.

Grey said the secretive proceedings, without due process, constitute "a direct assault on the basic principles of the Constitution ... It's a complete miscarriage of justice."

The NDP will apply, most likely to the Federal Court, for judicial review of the board's order that MPs repay $36,000 in Commons resources, Grey said. A separate court challenge will be launched if Canada Post demands repayment for the free mailing privileges, he added.

The party could yet wind up launching a third court challenge if the board orders New Democrat MPs to reimburse the salaries of staffers who were on the Commons payroll although they worked in NDP satellite offices in Montreal, Quebec City and Toronto.

Duncan said the board has not yet reached a conclusion on that issue.

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The courts have typically refused to intervene in the internal affairs of Parliament, which is master of its own house. But Grey said even Parliament must be "subject to the rule of law."

"It is a type of question, raising fundamental issues of our Constitution, which could well end up in the Supreme Court," Grey said.

Earlier Wednesday, NDP Leader Tom Mulcair predicted that the board would find his MPs guilty and order repayment.

"We're taking it for granted, frankly, that they're going to come out against us," he said.

Mulcair said the NDP would not repay money deemed owing by such a tainted process.

Duncan has said the Commons could garnishee the budgets of New Democrat MPs if they don't voluntarily repay.

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As he has done repeatedly since controversy erupted over NDP mailings and satellite offices several months ago, Mulcair insisted New Democrats have followed all the rules.

The Speaker of the Commons, Andrew Scheer, and House of Commons administration have contradicted NDP claims that they approved the mailings or the use of MPs' office budgets to pay staffers in satellite offices.