The NDP could be asked to repay potentially millions of dollars worth of mail it sent out into dozens of ridings on the taxpayers' dime, in a bitter dispute that has the party questioning the integrity of the Speaker of the House of Commons.

The closed-door committee of MPs that examines matters of House of Commons spending found Monday that a number of NDP members had broken the rules around the use of parliamentary resources, sources close to the meeting told The Canadian Press.

The Board of Internal Economy had been looking at up to 1.8 million pieces of mail that had been sent into dozens of ridings using Commons envelopes and the free postage services available to MPs.

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Conservatives and Liberals on the committee together found that the mail was partisan in nature, and therefore contravened the bylaws that govern spending by MPs' offices. Some of the flyers, printed by an outside printing company, carried political-style messages that portrayed the NDP as a better choice than other parties. Some pieces directed recipients to a website where they could donate to the party.

The NDP were seen to be exploiting a loophole in the mailing rules that did not explicitly bar such mailings that were put in a sealed envelope, as opposed to other kinds of MPs' pamphlets.

"It's a clear statement that they were not following the rules," said one person familiar with the committee's finding, who spoke on condition of anonymity.

The issue of a satellite office the NDP set up in the Montreal area using Commons resources is next up on the agenda for the committee.

"This is going to get worse before it gets better."

Who exactly will be asked to repay the money and how is unclear — dozens of MPs were involved in the mailouts, with NDP Leader Tom Mulcair's office responsible for a large proportion.

The amount of money at issue could be staggering. Each piece of mail would carry with it postage costs, plus the cost of each envelope. Commons legal staff are working out the bill, and options for how it could be repaid.

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Technically, each MP whose name was attached to a piece of mail could be responsible for repayment. But options that would include reducing the NDP's Commons budget, or having the party pay, are also being examined.

The NDP lashed out angrily at the committee in a late statement, calling the committee a "kangaroo court."

The party maintains that it had received formal clearance from the House of Commons administration and Speaker Andrew Scheer to make sure their mail was appropriate.

The statement went on to suggest that Scheer has been strongarmed by his colleagues in the Conservative party into suggesting that the NDP had not checked in advance.

"The Official Opposition now believes the integrity of the Speaker's chair — and the democracy it protects — is at risk," reads the statement.

Such accusations are rare in the Commons, where the Speaker is generally regarded as non-partisan and criticism rarely directed publicly to his chair.

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Commons Speaker Andrew Scheer is expected to make a statement about the matter Tuesday morning.