LOS ANGELES, Nov. 8 — At least two major television studios, 20th Century Fox and CBS Paramount, have sent breach-of-contract letters to the show runners on their current series who have stopped performing their production duties once they went on strike with other television writers.

The move is an escalation of hardball tactics by the studios. This week, the studios said they expected that the show runners — the writer-producers who oversee some of the biggest hits on television — would continue to work on the shows by performing nonwriting duties.

But after many of the industry’s top show runners said publicly that they did not intend to do any work as long as the strike by members of the Writers Guild of America continued, the studios began notifying the writer-producers that they would no longer be paid as producers if they failed to show up at work.

CBS Paramount — the studio behind hits like “CSI” and its spinoffs and “Rules of Engagement,” which are broadcast on CBS, and “Medium,” on NBC — began sending the letters this week after some show runners did not show up for work once the writers’ strike began on Monday, according to a person involved in the production of network television shows who spoke on the condition of anonymity.