The Canadian Union of Postal Workers and Council of Canadians have asked Ontario's Superior Court of Justice to declare Chapter 11 of the North American Free Trade Agreement null and void.
The NAFTA critics claim the provisions threaten Canadian sovereignty. Arguments began this week.
The suit came in reaction to a $200-million claim against the Canadian government by United Parcel Service. UPS claims subsidies for the Canadian mail system give Canada Post an unfair advantage over private companies. The union and the council fear that if the UPS claim is upheld, Canada Post would be unable to maintain services.