The founder and largest shareholder of Papa John's International sued the pizza chain's board and chief executive late on Thursday to stop what he described as "the irreparable harm" they are causing the company, according to a court filing.

John Schnatter has been in an escalating battle with the company's board since he stepped down as chairman in July after he said reports that he used a racial slur during a media training call were true.

Schnatter is suing the board and CEO Steve Ritchie "to stop the irreparable harm those individuals are causing due to their repeated, and ongoing, breaches of the duties of loyalty and care they owe to the company," according to a statement from Schnatter.

Papa John's did not immediately respond to a request for comment.

The lawsuit filed was under seal in Delaware's Court of Chancery, and Schnatter asked the court to expedite the case.

On Thursday, the company criticized Schnatter publicly for attending a meeting in July about a potential deal with hamburger chain Wendy's without Ritchie present, as the board had instructed.

Shares of Papa John's were up 0.9 percent at $46.50 on Friday.