Photo: The Canadian Press

A vow by British Columbia's Liberal leader to fight American tariffs on softwood lumber may force the federal government to wage an unwanted battle with the United States, an international trade expert says.

Christy Clark asked Ottawa to ban thermal U.S. coal shipments passing through B.C. ports after the Americans imposed an average tariff of 20 per cent on Canadian softwood lumber this week.

Clark said Friday on the campaign trail for the May 9 provincial election that if the federal government, which has jurisdiction over ports, fails to stop American coal shipments going from B.C. to countries such as China, she would impose her own measures.

"We can put a levy so onerous on the movement of thermal coal from British Columbia that no one will do it," she said in Burnaby.

However, Werner Antweiler, an associate professor who teaches international trade at the University of British Columbia's Sauder School of Business, said Clark's actions would violate North American Free Trade Agreement rules and "make things worse for the federal government" as it defends the province.

He said that as lead negotiator with the United States on a potential softwood lumber deal, Ottawa is in charge, not the provinces.

"If (the federal government) can't control what the individual provinces do, it diminishes their bargaining power," Antweiler said.

A forced dispute by Clark may also leave the federal government vulnerable to financial compensation, such as in previous cases involving other provinces.

Chapter 11, a controversial settlement clause in NAFTA, allows investors to sue foreign governments without first taking court action.