The letter was sent by Quebecor president and CEO Pierre Dion.
It asks the Liberal house leader Jean-Marc Fournier to leave Quebecor out of partisan debates at the National Assembly, after the Liberals pointed out last week that Quebecor-owned Videotron had higher interest rates than Hydro-Québec.
At a hastily-called news conference yesterday, Fournier distributed copies of the letter to reporters.
"This letter is an illustration about the fact that there is no separation at all between Péladeau and Quebecor," Fournier said.
Péleadeau, who gave up his management role at Quebecor, remains a controlling shareholder.
He responded to Fournier's allegations, saying he isn't surprised by the latest attack from the Liberals.
"I invite them to think about it because there could be politically harmful consequences for this kind of perspective," he said.