Chipotle Mexican Grill Inc, which has been under pressure from activist shareholder Bill Ackman, said on Friday four of its 12 directors would not stand for re-election to the burrito chain's board at its upcoming shareholder meeting.

John Charlesworth, Patrick Flynn, Darlene Friedman and Stephen Gillett will opt out of re-election this year, Chipotle said in a regulatory filing.

Friedman is the Denver-based company's longest-serving director with a 22-year tenure. Charlesworth and Flynn have served on the board for nearly two decades, while Gillett has been a director for about two years.

Chipotle said in December it had appointed four new members to its board, including one from Ackman's hedge fund Pershing Square Capital Management.

With a 10 percent stake as of March 3, Pershing is Chipotle's largest shareholder, according to the company.

Chipotle, once majority-owned by McDonald's Corp, has faced shareholder calls for an overhaul after a string of food safety stumbles that bruised its sales and reputation.

The restaurant chain was linked in November 2015 to a multistate E.coli outbreak, and the next month to a norovirus outbreak that sickened at least 80 Boston College students.

Following the food-safety lapses, which battered the company's shares and racked up high legal bills, Chipotle abandoned its dual-CEO structure and named Steve Ells the sole chief executive.

Reuters (Reporting by Richa Naidu in Bengaluru; Editing by Sai Sachin Ravikumar)