The TPP is a proposed agreement between not only the 12 Pacific Rim countries at the table, but others who could join later - countries like China, India or the Philippines.
In June, the prime minister called the deal "essential" for Canada.
Although TPP negotiations are held in secret, the publication Inside U.S. Trade reported Japan wants a rule that vehicles can be imported tariff-free to North America with just 30 per cent of a vehicle's content having been made in the U.S., Canada or Mexico.
Current rules mandate about 60 per cent of the vehicles be made in one of those three countries.
"Watering down the content thresholds amounts to opening a huge back door to our market for products made in China and other non-TPP countries," Unifor national president Jerry Dias said in a statement.
New world market
The Canadian government claims the deal would allow Canadian and American companies and manufacturers, automakers included, access to 800 million new customers.
Unifor Local 444 president, Dino Chiodo, who represents nearly 5,000 workers at Fiat-Chrysler's Windsor Assembly Plant says the auto industry will have trouble tapping into that market.
"When you look at Japan, currently, we don't ship hardly any vehicles to Japan because of the challenges that we already face there, with regards to the regulations they impose, with regards to the devaluation of their currency, with regards to challenges on how we can even get that vehicle to Japan to market because they don't have the same type of delivery process that we've established for that vehicle throughout the United States," he said. "Those are just a couple of the challenges that we're faced with."
Canada already incurs an annual $5 billion deficit in bilateral automotive trade with Japan, Unifor economist Jim Stanfield said earlier this week.
Unifor not alone
Unifor isn't alone when it comes to concern about Japan's demands.
Auto parts makers in Canada, the U.S. and Mexico have urged their governments not to give in to Japan.
Flavio Volpe, president of the Automotive Parts Manufacturers' Association of Canada, wrote Trade Minister Ed Fast to voice his association's concerns.
"An inadequate content rule for parts will dramatically impact our domestic parts manufacturing industry and the jobs that come with it," the letter said.