The NDP is calling on Commons Speaker Andrew Scheer to review the way the ultra-secret Board of Internal of Economy is handling a series of investigations into the party and its use of parliamentary resources, CBC News has learned.

In letters obtained by CBC, NDP MP Craig Scott asks Scheer to look into some 20 direct mailings sent by Conservative and Liberal MPs.

Scott, the NDP's democratic reform critic, suggests the other parties have been "immune" from board scrutiny because the Conservatives and Liberals formed an alliance to condemn their NDP counterparts.

The powerful Board of Internal Economy oversees the budget of the House of Commons and is made up of four Conservative MPs, two NDP MPs and one Liberal. As Speaker, Scheer is chair of the committee.

In June, the board requested the Official Opposition pay back $1.17 million for mailings deemed too partisan to have been sent out with taxpayer money. The board said the NDP should repay $36,000 to the House of Commons for misuse of parliamentary resources and more than $1 million to Canada Post, though it has no way to force Canada Post to go after the money.

Scott says in his letter that the board has "refused all requests to formally justify the cryptic condemnation of the NDP MP mailings."

Political battle heats up

The NDP is already fighting the matter in court, and in legal filings has said the decision by the board was "unreasonable and incorrect."

The letters to the Speaker will inevitably put the controversial issue front and centre again as Parliament is set to resume next week. They also suggest the NDP is ready to continue this battle on the political front.

Scott says the NDP has now found breaches by the other parties of the same rules the NDP is accused of breaking.

Scott says a handful of Conservative and Liberal MPs sent partisan mailings in House of Commons envelopes, that those mailings violated the House's own by-law on "prominence" of the MP's name, and that the requirement to have one designated and approved website on the mailings was also not met.

The NDP suggests it all points to an unfair process.

"In light of previous BOIE rulings, and without prejudice to legal actions undertaken by NDP MPs, I am calling on the BOIE, through you, to explain why it has ignored and effectively acquiesced in the practices described above…," Scott writes.

Scott writes that given the nature of the mail-outs, the examples he cites are likely just the "tip of the iceberg."

In a separate letter, NDP MP Robert Aubin says the NDP has also been treated unfairly on the issue of satellite offices, calling that process "arbitrary and illegal."

In that case, the board found the NDP also misused House resources when it allowed parliamentary staff to work out offices rented by the party in Montreal.

In the letter, Aubin demands the board release evidence of what rules the NDP broke and to provide the evidence.