Over two dozen Mos Burger customers came down with food poisoning last month after dining at its restaurants in Tokyo and seven other prefectures in eastern and central Japan, it has been learned.

Twelve of the 28 diners sickened were found infected with the O-121 strain of E. coli bacteria, the Health, Labor and Welfare Ministry said Friday.

"It is highly likely this was caused by foodstuffs supplied by the headquarters," the chain's operator Mos Food Services Inc. said.

Those affected dined at 19 restaurants in Tokyo and Saitama, Kanagawa, Chiba, Gunma, Tochigi, Yamanashi and Nagano prefectures between Aug. 10 and 23, Mos Food said.

A Mos Burger in Ueda, Nagano Prefecture, was closed for three days through last Wednesday after being ordered to do so by the public health office, the company said.

Mos Food said Saturday that it has apologized to the victims and their families.

"We'll step up our sterilization of vegetables, which are believed to be a cause of the health problem," a public relations officer at the company said.