The committee said it needs more time and information before deciding whether the area should be deemed a world heritage site.
The decision was a blow to the Manitoba government, which has already spent or committed to spend $14.5 million on Pimachiowin Aki, a 33,400-square kilometre swath of pristine boreal forest. The Ontario government has put up a smaller amount of money through annual allotments.
The Manitoba government said it will keep pursuing the designation.
"There may be some more investment needed in order to facilitate the mission and work with the partners on that one, so we'll have discussions and work out a work plan with our partners ... very quickly," Conservation Minister Gord Mackintosh said.
The two provinces have been working toward the UNESCO designation for several years, along with five First Nations that want to protect the remote area. Its name is an Ojibwa phrase that translates as "the land that gives life." The area is touted as the largest intact section of northern boreal forest. Its lack of roads and other infrastructure have kept it relatively unchanged over the centuries.
The governments and First Nations submitted a bid to UNESCO last year and were hoping to get approval at next month's annual meeting of the agency's world heritage committee in Cambodia. But an advisory body to that committee said Friday there are still unanswered questions as to the region's uniqueness.
"Although (the) association between people and nature is strong in Pimachiowin Aki, what has not been demonstrated is how this association can be seen to have outstanding cultural value within their geocultural area," the advisory group wrote.
It raised other concerns as well and called for a second visit to the region by UNESCO officials, who made their first trip last fall. That means the bid won't be reconsidered until at least 2014.
Manitoba is struggling with a half-billion-dollar deficit, but Mackintosh said the millions being spent on getting a UNESCO designation are well spent
"There are multiple benefits from a world heritage site. First of all, a recognition that here, in this part of the world, we have something very unique in terms of culture and ecology.
"More significantly, this is about (environmental) protection."
The government has also suggested that a UNESCO designation would be a way to attract tourists. But a study done for the project estimated that it might attract fewer than 1,000 tourists a year, largely because of its remoteness.
"The type of tourism development appropriate for Pimachiowin Aki is a niche market, attracting hundreds, or possibly as much as a few thousand, visitors per year," said the study prepared by Marr Consulting Services.
"Projections should be conservative in estimating revenues from tourism."
That view is also held by Elizabeth Halpenny, an assistant professor of wilderness studies at the University of Alberta.
"There are what we call hard-core ... collectors — people who collect world heritage sites. That's how they make their destination decisions or their vacation decisions," Halpenny, who specializes in nature-based tourism, said in a recent interview.
"But the world heritage brand is an important question. Is it an attraction to the everyday person, the person who might be travelling? Likely, a small percentage in each country might recognize the brand."
Halpenny added the need for tourists to fly to the region would make a visit expensive and there is already a lot of competition for the eco-tourist dollar.
There are already more than 900 places in the world that are UNESCO world heritage sites — everything from the Great Barrier Reef in Australia to the town of Lunenburg, N. S. to an old deciduous forest in Belarus.
Manitoba's NDP government has faced criticism over its decision to protect the area in and around Pimachiowin Aki from hydro development. In 2007, the government prevented Manitoba Hydro from building a transmission line through the region down the east side of Lake Winnipeg. It ordered the Crown utility to build a much longer line to the west of lakes Winnipeg and Manitoba, which will cost at least several hundred million dollars more.
The Opposition Progressive Conservatives denounced the move as costly and unnecessary. They have repeatedly said construction of a transmission line wouldn't hurt the chances of the vast forest being declared a world heritage site.