FamilyMart has denied accusations that it sold expired food after a video came to light claiming that 21 of its convenience stores in 10 cities across the country did so.

The video, released by review website Lanmei Testing - which is known for its expose of unhygienic practices at luxury hotels in 2017 - received immediate public attention on Tuesday soon after it was posted on Chinese social networks Sina Weibo and WeChat.

On Weibo, the video accumulated nearly 10,000 comments within hours, while on WeChat the article containing the video had received more than 100,000 page views as of Tuesday evening.

In response, FamilyMart's China operator, Shanghai Fumanjia Convenience Co, issued a statement on Tuesday afternoon saying the company has strict rules prohibiting sales of expired food and has inspected the stores mentioned in the video and found no such offenses. Instead, as it checked surveillance footage around the time when Lanmei Testing shot videos at their stores, it said, some "unusual" purchases were discovered that might have ended up in the video.

In an online briefing session with the media, Wang Yiwen, the company's public relations manager, said the company has strict internal management rules for removing expired food, and that it might resort to the courts to defend its reputation.

FamilyMart is one of the most popular convenience store brands in China, with more than 2,500 stores.

Xu Penghao, chief editor of Lanmei Testing, said the website decided to investigate FamilyMart after one of its staff members bought an expired sandwich at a store he frequented in Beijing. Xu said when the customer went back to confront the convenience store manager, the manager showed no surprise and offered two cups of free coffee as compensation, which made him suspect that the sale of expired food was a common occurrence.

From November to December, Lanmei Testing said, it visited 100 FamilyMart stores in 10 cities, including Beijing, Shanghai and Guangzhou and discovered expired food, especially bread and cakes, on shelves in 21 stores.

Its video included footage of an investigator purchasing a cake that was one day past the expiration date at a FamilyMart store on Wangfujing street in Beijing on Nov 14.

Lanmei Testing said it has preserved all its recordings taken in one shot and vouches for their authenticity.

Many of the netizens commented that they have experienced similar issues in different convenience stores, including 7-11 and Lawson.

"We are not targeting anyone, but want to raise the attention of all convenience stores to make food safety a top priority," Xu said.

Chinese news website Thepaper.cn reported that the Shanghai municipal market supervision bureau was investigating the case.

In a FamilyMart store on Huaihai road in Shanghai, staff members said they had received instructions to double-check the shelves for any expired food.