Opposition to Canadian oil and gas pipelines has grown into an international environmental protest industry from a modest 2008 beginning as “The Tar Sands Campaign,” according to an Alberta government inquiry into foreign funding of home-grown fossil fuel foes.

The 657-page Report of the Public Inquiry Into Anti-Alberta Energy Campaigns found that C$1.28 billion ($1 billion) in documented cross-border support for the resistance movement is “likely understated” given slack disclosure rules.

Inquiry commissioner J. Stephens Allan wrote that the two-year, C$3.5million ($2.8 million) investigation detected no illegal activity. However, he stressed that campaigning nonprofit groups and charities contend with far fewer regulations than oil and gas pipeline companies.

“The...