In an October report, the Federal Review Panel found that Taseko's New Prosperity Mine would have "significant adverse environmental effects."
The panel specifically referenced the negative impacts that seepage from the mine's tailings would have on water quality and fish habitat in nearby Fish Lake.
The proposal was rejected by the Ministry of Environment once before, following similar conclusions by the review panel in 2011. Plans for conserving Fish Lake were a large part of Taseko's revised proposal submitted this year.
Taseko vice president Brian Battison says the review panel based their conclusions on an incorrect design of the tailings facility provided to them by Natural Resources Canada, an error the company called so "outrageous as to be nearly unfathomable."
"It's unlikely that an error of this magnitude has been discovered before in an environmental assessment," Battison told CBC News.
Taseko officials sent letters to Minister of the Environment Leona Aglukkaq and vice president of the Canadian Environmental Assessment Agency Yves Lebeouf earlier this month that outlined the error.
In response, the CEAA asked that Taseko provide evidence for their claim that an error was made and provide the correct information to the agency.
Taseko claims that they have fully complied with the CEAA's request, but that the information provided has not been made public and that could affect the final decision on the mine proposal.
"It's very important, this piece of information, because it's all about the relationship between the tailings facility, the lake and the mining operation. This whole plan was designed to save Fish Lake, and it costs us three hundred billion dollars," says Battison.
"It was a different plan. They mistakenly concluded the project would contain significant environmental affects."
The CEAA did not respond to inquiries from CBC News.
The federal environment minister is expected to make a final ruling on the proposal by March 2014.