OTTAWA -- The head of an agency that has spent years shining a light for Canadians on the sometimes-opaque world of government spending is slamming the Trudeau government for its plan to give his office a makeover.

Parliamentary budget officer Jean-Denis Frechette laid out detailed arguments Wednesday on how proposed legislation would limit the freedom and capabilities of an office with a track record of getting under the skin of governments.

Frechette's office -- also known as the PBO -- is designed to serve parliamentarians as a check on the management of the nation's finances.

"Those restrictions will undermine PBO's functional independence and its effectiveness in supporting parliamentarians to scrutinize government spending and hold the government to account," Frechette said Wednesday in a statement that accompanied a discussion paper on the legislation.

The government has faced criticism since proposing the changes last month and, following the backlash, the Liberals have said they are open to tweaks.

Frechette said the most concerning proposals include new controls over the PBO by the House of Commons and Senate Speakers, limits on the office's freedom to initiate reports and restrictions on individual parliamentarians' ability to request estimates.

Among the changes, the legislation would require the PBO to submit its annual work plans to the Speakers of both parliamentary houses for approval.

In addition, the law would prevent the PBO from making a report public until a day after it is provided to the Speakers or to the parliamentary committee that requested the research.

Frechette, who says he wasn't consulted on the legislation, also outlined the risks associated with an added mandate for the PBO to cost election pledges by political parties.

It would be the "most significant departure" from its current role, he noted.

Frechette said costing election vows for political parties would turn the office into a research bureau for policy development, seriously undermine its perceived independence and deplete its resources.

He told a news conference Wednesday that he met with high-ranking officials from the Privy Council Office last week to learn more about the motives behind the bill.

Frechette said he was "furious" and then "puzzled" when told the government wanted to "reset the sometimes dysfunctional relationship with the PBO." He added that a recent focus group poll showed the office is in good standing with parliamentarians.

Assistant parliamentary budget officer, Mostafa Askari, also attended the meeting. He said PBO officials were also told that their office had "a lack of focus on services to Parliament."

"First of all, this is false -- completely," Askari said.

"It's also insulting, to be honest, to those who are working within PBO and those who've worked for PBO in the past."

Askari said he believes the push for changes to the PBO has been driven by members of the public service, whom he alleged have seen the office as competition since its creation in 2008.

He estimated that between 30 and 40 per cent of PBO studies would not have been possible over the past nine years, if the proposed changes had been in place.

For example, the office said its high-profile examination of the costs of acquiring F-35 fighter jets would have been among many notable studies that would have never been conducted.

Askari said while the legislation includes many "poison pills," he's encouraged the government has indicated publicly that it's open to amendments.

"Maybe they have realized there are some problems," Askari said.

A government source says the feds are considering amendments to ease a new requirement that would force the PBO seek approval from the Speakers and the removal of rules restricting which subjects it may study.

Prime Minister Justin Trudeau argued this week during question period that the changes would give the PBO more resources and stronger autonomy. His comments came after NDP Leader Tom Mulcair accused him of "muzzling" the office.

The legislation would make the PBO an independent Officer of Parliament and provide it with expanded access to data.

Government House leader Bardish Chagger's office has insisted there are many benefits from the proposed reform, such as a change that would set the budget officer's term at seven years and another stating he or she could be removed from the role only through a vote by both chambers.

During the 2015 election, the Liberals pledged to ensure the PBO "is properly funded, and accountable only -- and directly -- to Parliament, not the government of the day."