The woman appointed to probe last summer's deadly listeriosis outbreak has a glaring conflict of interest and can't credibly assess how public safeguards failed, critics say.

The Liberals, New Democrats and Bloc Québécois are all pushing for a full judicial inquiry into the disaster linked to Maple Leaf Foods.

Prime Minister Stephen Harper stopped short of that as he named Sheila Weatherill this week to lead an "arm's-length investigation."

He appointed the former Edmonton health care executive even though Weatherill already serves on the prime minister's advisory committee to revamp the public service.

Its mandate includes: "Branding the public service as a trusted and innovative institution of national importance."

Critics are asking how Weatherill can do that job while leading a probe into whether food safety agencies broke the public's trust.

"I think it's pretty clear: Ms. Weatherill can be a cheerleader for the public service, or she can be an independent investigator of the public service," said University of Ottawa researcher Amir Attaran, a lawyer and biologist by training.

"But she can't be both at the same time. In ignoring that reality, Mr. Harper has foolishly failed once again to keep conflicts of interest out of his government."

Attaran co-signed an editorial in the Canadian Medical Association Journal in the fall that charged that "government policy errors helped bring about" the listeriosis outbreak.

Changes to government monitoring mean Canada now has some of the lowest listeria standards among developed countries, it said. It demanded a full public inquiry into Canada's food inspection system.

A "proper inquiry, convened under the Inquiries Act and with a judge sitting as a commissioner of inquiry, is needed more than ever," Attaran said Wednesday.

Twenty people died after developing listeriosis — a particular threat to the elderly, pregnant women, and those with fragile immune systems.

Weatherill is to assess what went wrong, how federal food-safety and recall systems responded, and make recommendations to Agriculture Minister Gerry Ritz.

Her report is due July 20. That's four months past the original March 15 due date outlined when Harper first promised the probe last September, a few days before calling an early federal election.

The prime minister's office says Weatherill won't publicly comment until her report is finished. The government will then decide what details to release.

Harper's office has deflected criticism for the delay in appointing Weatherill, saying it took time to find the right person for the job. Spokesman Kory Teneycke also dismissed Wednesday suggestions that she is in conflict.

"Quite the opposite," he said. "We think that Ms. Weatherill's commitment to strengthening the public service and attracting top-level people…is actually very consistent with her investigating how to improve the processes in government — both at the federal and provincial levels — and make recommendations so that if a similar situation were to occur in the future that the government response would be even better."

Liberal MP Wayne Easter said the Commons agriculture committee should investigate the listeriosis outbreak if Harper won't budge. His NDP and Bloc Québécois counterparts agreed that a transparent process with the power to compel sworn testimony is needed.

Of particular interest is the extent to which the Prime Minister's Office may have tried to micro-manage the Canadian Food Inspection Agency, which is supposed to be arm's-length, Easter says.

"The fact of the matter is her credibility has already been compromised in her ability to do the job," Easter said of Weatherill. "She will certainly be perceived…as a friend of the prime minister."

Weatherill's selection as a standard-bearer for public-service reform raised eyebrows for other reasons.

Alberta's auditor general took aim last October at some of the million-dollar salaries paid to heads of regional health boards, including Weatherill.

She earned $915,000 a year as head of Edmonton's Capital Health until she and seven other top executives lost their jobs last July. Their dismissals were part of the provincial Conservative government's move to integrate separate health regions into one super board.

Auditor General Fred Dunn was harshly critical of the health boards that approved such salaries.

"How and why did they arrive at this level of compensation?" he asked at the time. "In some cases, I don't believe it was as robustly negotiated as it should have been."

Weatherill was paid almost $3.5 million in a severance and retirement package under those contract terms.