The NDP is claiming it is a victim of a "kangaroo court" after it was determined that the party broke the rules around use of parliamentary resources by sending out at least 1.8 million pieces of partisan mail to households across Canada.

At a news conference Tuesday afternoon, NDP House Leader Peter Julian defended the party, saying that the Liberals and Conservatives are banding together and unfairly attacking the official opposition.

Julian appeared alongside fellow New Democrats Nycole Turmel and Pat Martin, with Martin alleging the Conservatives had sent out partisan mailings using House of Commons envelopes to constituents in his riding.

Martin said the NDP approached the Board of Internal Economy with the Conservative mailings to check if they had broken the rules, and the board said they were OK.

Julian said this suggests that the Liberals and Conservatives are targeting the NDP.

"What's happening is a partisan attack on one political party to the exclusion of all others," he said. "The Conservatives are saying, 'No matter how partisan we get, it's OK. We set the rules, we're judge, jury and executioner.'"

The three MPs spoke hours after the BOIE said it would be seeking advice on "appropriate remedies" after it had concluded that NDP MPs contravened rules on using parliamentary resources. The BOIE is the closed-door committee that oversees Commons spending.

Speaker of the House of Commons and board chairman Andrew Scheer said in a statement Tuesday that the board has also asked that the NDP provide proofs of additional mailings by June 13.

Earlier Tuesday morning, Conservative MP and board member John Duncan wouldn't give an estimate on how much money the NDP may be asked to repay, noting that he didn't want to prejudge the process.

The board had been looking at up to 1.8 million pieces of mail that the NDP had sent to 26 ridings using Commons envelopes and free postage services used by MPs.

The so-called franking privilege allows MPs to send postage-free mail to constituents, but they are not allowed to use parliamentary resources for electoral campaigning.

The mailings had the NDP logo on them and directed readers to the party's website.

The board had determined Monday that the NDP broke the rules when it used taxpayer-funded resources to send the mailings out to several different ridings.

According to Scheer's statement, the mailings were "prepared by and for the benefit of the NDP as a political party and to advance electoral purposes, and are therefore in contravention of the by-laws."

The NDP maintains the mailings were cleared by House of Commons clerk, Audrey O’Brien.

Despite the allegation that O’Brien gave permission to mail the letters, the NDP has been opposed to hearing testimony from O’Brien at committee. She is expected to speak on the matter on Thursday.

In a statement Monday, the NDP said they believe the “integrity of the Speaker’s chair – and the democracy it protects – is at risk.”

Duncan rejected the NDP’s accusations that the board was acting unfairly.

"We've been very fair. They should be accepting responsibility instead of impugning everybody else, including the Speaker," he said. "I do believe at this point that they should be apologizing to the Speaker, who they know darn well is in no position to defend himself. That would demean his office."

Duncan said the board could seek different ways to recover the money, in the event the NDP is asked to make a repayment but refuses.

With files from CTV News’ Laurie Graham