SHANGHAI (Reuters) - United States fast-food chain Subway is investigating media reports in China that workers at an outlet in Beijing doctored food labels and used produce beyond its expiry date, a Shanghai-based spokesman for the company said on Monday.

Chinese media reports that started circulating on Friday said workers at a Beijing outlet for Subway, which operates globally as a franchise business, changed expiry and production dates on meat, drink and vegetable produce to extend their use. "Our headquarters here is now investigating the matter," a Subway spokesman said. She said the company had not reached any conclusions. "We want to investigate what caused the labelling issue and whether or not it was the action of a single franchised outlet."

The Subway spokesman said the firm sent teams to inspect its franchises around the country each month and that it gave staff training on food safety and handling produce. The Beijing food regulator had also launched an investigation, she added.

The issue highlights the difficulty companies have to control food quality and safety in supply chains and outlets in China, which can cause serious headaches with regulators and hit sales.

Earlier this year, a China-based food supplier to McDonald's Corp and Yum Brands Inc was caught up in a scandal over workers allegedly tampering with expiry dates, which pummelled China sales at the fast-food chains.

Food safety scandals in China are all too common, including recycled "gutter oil" for cooking, donkey meat products tainted with fox and one in 2008 involving tainted dairy products that led to the deaths of at least six infants.

Subway, part of Doctor's Associate Inc, has close to 500 outlets in China, according to its official website.