The NDP and Parliament's budget watchdog are asking the Federal Court to clarify his mandate and whether he has the power to ask for specific financial information from federal departments.
It's the next step in a longstanding dispute between parliamentary budget officer Kevin Page and the Conservative government.
The Conservatives have argued he is overstepping his mandate by asking for specific information about federal budget cuts, such as the number of jobs being chopped and how much service in each department will be affected.
It's expected the court case could move quickly because both the NDP and Page want to expedite the legal opinion.
Earlier this month, NDP Leader Tom Mulcair requested an analysis by Page over whether the savings projected in the last budget can be achieved and whether they will have effects on Canada in the long term, according to letters contained in documents filed in Federal Court.
Page replied that "questions have been raised as to whether the analyses you require fall within my mandate" and that he would need to know "the departmental savings premised on staffing reductions."
In a subsequent letter, Page wrote he would ask the court whether it's in his jurisdiction to analyze the extent to which the savings projected in the budget can be achieved, the extent to which that would have "fiscal consequences" in the long term and whether it's in his jurisdiction to request departments give him the savings they calculated based on staffing reductions by program activity.
"If the court decides that I do not have jurisdiction to perform the analysis you require, it will not be performed and the data will not be requested. If the court decides that I have jurisdiction to perform the analysis you require and request the necessary data, I will ask for the data and, when provided with it, endeavour to produce the analysis," Page wrote to Mulcair.
Conservatives aim to 'shut down' Page
Mulcair says the government has made it clear that it will try to try to control Page, despite boasting about creating his job when it first took over.
"They [the Conservatives] started playing with his budgets, they started making it difficult for him to hire and retain staff," Mulcair said outside the party's weekly caucus meeting.
"That's the game the Conservatives have played since Day 1. But it's a constant with the Conservatives. Anything that dare stand up to them, that doesn't tell them exactly what they want to hear will be shut down. That's the constant message from the Conservatives."
Finance Minister Jim Flaherty, leaving his own party's caucus meeting, repeated the government's assertion that Page was stepping outside his role.
"He should stick to his mandate," Flaherty said.
In the House of Commons Wednesday, Prime Minister Stephen Harper said the government had made all "relevant" information available.
"We created the office of the parliamentary budget officer so he could do his non-partisan work and we'll continue to supply information for that non-partisan work."