Quebecor boss Pierre Karl Peladeau made the threats in letters to Radio-Canada president Hubert Lacroix, including one sent last September.
A Quebecor spokesman said this week the company is in the final stages of analyzing the dossier.
Lacroix has responded by saying Quebecor's readers aren't part of the target audience sought by the public broadcaster.
The correspondence between Peladeau and Lacroix was obtained by The Canadian Press under the Access to Information Act.
Quebecor (TSX:QBR.B) owns TVA, the French-language broadcaster that is Radio-Canada's biggest competitor in Quebec, as well as Sun Media.
In Quebec, its stable of publications include Le Journal de Montreal, a wildly popular tabloid.
The letters between the two men have flown since at least August 2009, although the latest documents ramp up the tone of Quebecor's arguments considerably.
Peladeau concludes that Radio-Canada stopped buying ads in Quebecor publications when it locked out employees at two newspapers.
He says Radio-Canada is also reacting to criticism of the Crown broadcaster by Quebecor journalists.
The warning of legal action came in a missive sent last Sept. 7.
''If you continue harming Canadians by pursuing these illegal measures, we are advising you that we will ask the courts to stop these petty and illegal practices," Peladeau writes.
The warning was repeated in another letter in December, although Peladeau did send New Year's greetings to Lacroix on the Quebecor website in which he said it did not appear as though Lacroix had made a resolution to ''eliminate his discriminatory advertising practices."
Lacroix has consistently insisted in the correspondence that Quebecor's market doesn't reflect what Radio-Canada is seeking.
"The choice of advertising vehicles to promote Radio-Canada's programming is done without regard to ownership considerations," he wrote Peladeau on Sept. 29, 2009.
"Its choice of advertising space is aimed only at meeting the requirements to promote its programming."
Lacroix makes the same point in several letters.
Quebecor has not gone to court yet.
"Quebecor Media's legal service is completing the analysis of the file," Serge Sasseville, the company's vice-president of corporate and institutional affairs, said this week.
"Because we're in the last phase of our analysis, it would not be appropriate for us to comment."
Radio-Canada insists its advertising strategy is perfectly legal.
"We have done nothing wrong," said spokesman Marco Dube.