Officials with the Jing'an District Market Supervision and Management Bureau check an outlet of FamilyMart inside the Jing'an Temple Station.

Two outlets of FamilyMart in Jing'an and Yangpu districts are being investigated by the market watchdog after they were accused of selling expired food, but the convenience store chain vows to fight back.

A video by social media Blueberry Test claimed it found expired food sold at 21 outlets of FamilyMart in 10 cities including Shanghai, Beijing and Hangzhou and uploaded a video on WeChat and Weibo.

The two stores in Shanghai accused in the video are the Jing'an Temple station outlet and theFudan University outlet.

At the Jing'an Temple station outlet, officials checked documents and stock on Tuesday afternoon. The food shown in the video was some blueberry chiffon bread sold in late November, according to the outlet workers.

"We are checking the product procurement and destroying records of the store and will make a review and investigation," an officialsurnamed Zhu, with the Jing'an District Market Supervision and Management Bureau, told Shanghai Daily. Officials did not find expired food at the scene.

The store underwent renovation in December and the surveillance camera footage before December could not be found, said a manager of the store surnamed Qian.

The involved bread has not been sold at the store for some time, he said.

FamilyMart said in a statement it has conducted a check on all stores involved in the video, and did not find expired food.

It said its surveillance camera footage captured suspected "abnormal" purchases by workers from Blueberry Test repeatedly, and will take legal action.