The operator of McDonald's in Japan said today that same-store sales plunged 21% in December from a year earlier after it was hit by a shortage of french fries. 

McDonald's Holdings Co (Japan) restricted sales of chips to small servings only from December 17 up to January 4, as a labour dispute at US West Coast ports delayed imports. 

In a new blow, the company also disclosed this week that customers had found objects including a human tooth and plastic in its products, reigniting safety fears. 

Monthly sales have suffered double-digit percentage falls since July following a food safety scandal at a Chinese supplier of chicken for its McNuggets. 

Sales at stores open at least 13 months slid a record 25% in August.