-- Hundreds of Chipotle restaurants aren't serving pork after the national chain suspended a supplier for violating its standards.
Disappointed customers found signs this week that read, "Sorry, No Carnitas" outside restaurants.
Chris Arnold, a spokesman for Chipotle Mexican Grill, said the company learned about the violation this past Friday through a routine audit and did not know when pork would return to its affected restaurants, about a third of them, according to the Associated Press.
"This is fundamentally an animal welfare decision, and is rooted in our unwillingness to compromise our standards where animal welfare is concerned," Arnold told ABC News.
The issues are mostly related to violations for the housing of pigs, Arnold said. As part of its "Responsibly Raised standard," Chipotle demands that suppliers raise the animals in humane conditions with access to the outdoors instead of tight pens.
As part of Chipotle's "Food With Integrity" program, the company says it has chosen meat from pigs that are "naturally raised," without antibiotics and are fed a vegetarian diet since 2001, according to its website.
"The standards for all of our Responsibly Raised meats are very high, but the differences in animal welfare are greatest with pork," Arnold told ABC News. "Given these stark differences, serving pork from conventionally raised pigs is not an option to us."
The company had 1,724 restaurants as of Sept. 30.
Chicken is actually the most popular topping at Chipotle, Arnold said, while the carnitas topping typically accounts for about 6 to 7 percent of entree orders.
The company has chosen to serve "naturally raised" beef since 1999, but sometimes faces limited supplies of antibiotic-free cows.
"Once-in-awhile we do experience a shortage in certain areas of the country and we’ll let you know at the restaurants if that happens," Chipotle says on its website.
As an alternative to meat, the company is marketing its vegan tofu topping Sofritas, introduced in July 2013, with a promotion.
Their new promotion can earn you a free entree just for buying one of their tofu-based meals. Buy a Sofritas entree on Jan. 26 and use the receipt to get a free entree of your choice between Jan. 27 and Feb. 28.
Arnold said the company is "exploring many options to add to our pork supply, including adding new suppliers, finding additional cuts of pork to use, and growing the supply among existing suppliers."