A public inquiry into the murder of a five-year-old Manitoba girl could be delayed by the union representing the province's child-welfare workers.

The Manitoba Government and General Employees' Union (MGEU) has filed a motion questioning the legal validity and jurisdiction of the provincial inquiry into the 2005 death of Phoenix Victoria Hope Sinclair.

The body of Phoenix Sinclair, seen here in an undated photo, was discovered in March 2006 on the Fisher River First Nation. A public inquiry into her death is slated to begin May 23. (Family photo)

Phoenix was killed on the Fisher River First Nation in June 2005, but it wasn't until March 2006 that her body was found wrapped in plastic and in an unmarked shallow grave.

The girl died after neglect and repeated abuse by her mother, Samantha Kematch, and stepfather, Karl McKay, both of whom were convicted of first-degree murder in December 2008.

The inquiry, which was ordered by the province in 2006, aims to examine how Manitoba's Child and Family Services officials failed to protect Phoenix and why her death went undiscovered for months.

The inquiry is set to begin on May 23, but that start date could be pushed back if the MGEU succeeds with its motion.

Jeff Gindin, the lawyer representing both Phoenix's biological father and her former foster mother, told CBC News he was served with the motion last week.

"The inquiry was called quite some time ago. A lot of people have been working very hard in preparing for the inquiry, including the commission counsel interviewing witnesses," Gindin said Sunday.

"MGEU has filed a motion some time ago, trying to restrict publication of the names of their employees who may testify," he added. "Now, with a few months to go, they file this motion."

Lawyers for the union will appear in a Winnipeg courtroom on Thursday to get a hearing date set.

Was in foster care

Phoenix had spent much of her young life in foster care before Kematch regained custody of her in 2004.

The public inquiry had previously been delayed by unsuccessful appeals by Kematch and McKay.

Last year, commission counsel said more time was needed to wade through piles of child welfare documents, all of which require court permission before they can be made public.

As for the MGEU's motion, Gindin said the public-sector union should have filed it months ago, when planning for the inquiry was in its earlier stages.

"From a strictly logical point of view, you would think someone who had a problem with the inquiry even taking place would maybe deal with that first and early on," he said. "I'm sure that someone's going to say that, and essentially that was the commissioner's response. We've been working hard at this for 11 months, and we're not going to just stop easily."

Neither officials nor lawyers for the MGEU responded to requests from CBC News for comment on Sunday.

The Assembly of Manitoba Chiefs and the Southern Chiefs Organization are holding a news conference on Monday to talk about the union's action, which both aboriginal groups say is "unnecessarily stalling" the inquiry.