Customer buys food at a Subway fast food restaurant in Beijing, China, Dec 5, 2013. [Photo/IC]

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.

Earlier this year, a food supplier in China for McDonald's and Yum Brands was caught up in a scandal over workers allegedly tampering with expiration dates, which hit sales in the country at the fast food chains.

The Subway spokeswoman said that the company sends teams to inspect its franchises around the country each month and that it gives employees training in food safety and handling of produce. The Beijing food regulator has also launched an investigation, she said.