US fast food chain Subway is investigating media reports in China that workers at a Beijing outlet doctored food labels and used produce beyond its expiration date, a spokeswoman for the company said in Shanghai on Monday.

Chinese media reports that started to appear on Friday said workers at the outlet changed expiration and production dates on meat, drink and vegetable produce to extend their use.

"Our headquarters here is now investigating the matter," the Subway spokeswoman said, adding that the company had not reached any conclusions. "We want to investigate what caused the labeling issue and whether or not it was the action of a single franchised outlet," she said.

The issue highlights the difficulty companies face in controlling food quality and safety in supply chains and outlets in China, which can cause serious headaches with regulators and affect sales.

The Subway spokeswoman said that the company sends teams to inspect its franchises around the country each month. The Beijing food regulator has launched an investigation, she said.