McGuinty, who was in office at the time of the disaster in Elliot Lake, Ont., and who called the probe, is likely to be on the witness stand either in late summer or early fall.
"He's part of the narrative," commission lawyer Mark Wallace said Wednesday from Elliot Lake.
McGuinty will only be part of Phase II of the inquiry — the examination of the emergency response to the collapse of the rooftop parking deck at the Algo Centre Mall that killed two women.
Questions have arisen about whether the overall emergency response to the unfolding tragedy was adequate.
In particular, many residents expressed concern the rescue effort was called off prematurely — just hours after signs of life were detected in the rubble.
After some form of intervention from McGuinty, emergency officials announced resumption of the rescue effort, which turned out to be in vain.
"It appears that there was a rescue stoppage or suspension, and he was involved in a phone call that resulted in resuming the rescue operation," Wallace said.
It's rare for an Ontario premier or former premier to be called as a witness at a public inquiry but not unprecedented.
In June 2001, Mike Harris became the first sitting Ontario premier in more than half a century to do so when he testified at the commission looking into the tainted-water tragedy in Walkerton, Ont.
Five years later, Harris also testified as a former premier at the Ipperwash inquiry into the 1995 death of an aboriginal man during a native protest occupation of a provincial park.
Wallace said McGuinty has been told he will be called to testify, but he has yet to be formally summonsed.
McGuinty, who stepped down in January but remains a member of the provincial legislature, could not be reached for comment Wednesday but confirmed in a statement he will be a witness.
"I am very proud of the work being done by the Elliot Lake inquiry," McGuinty said.
"I look forward to my contribution, and am confident the inquiry will provide all Ontarians with some very helpful recommendations."
Like all other witnesses, counsel lawyers will interview the former premier ahead of his testimony, something that hasn't happened yet.
The inquiry began last month and has been looking into the causes of the June 23 mall collapse that killed Lucie Aylwin, 37, and Doloris Perizzolo, 74. Several others were hurt.
Their bodies were pulled from the rubble on June 27.
The commission has so far heard from two dozen of about 64 Phase I witnesses.
Bob Nazarian, who owned the mall at the time of the collapse, is expected to testify in early July, the commission said Wednesday.
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