Wal-Mart Stores and Carrefour have apologised after being fined for using faulty price tags and overstating discount claims in China.

China's National Development and Reform Commission has said they would be fined together up to CNY500,000 (US$75,912) and income generated by these claims would be confiscated - ten Carrefour and three Wal-Mart stores are involved.

The commission's action follows customer complaints about overcharging on promotion items.

"We sincerely apologise for causing losses to the customers and will refund five times the difference," said Carrefour, claiming the issue had been caused by "defective price tag reading systems" now being upgraded. Carrefour added it would increase branch inspections.

Shirley Zhang, a spokesperson at Wal-Mart China, said: "We sincerely apologise for causing losses to the customers. Wal-Mart is now in co-operation with relevant government departments to further investigate the case, and will punish the staff involved."

Wal-Mart has established a special price supervision group to make sure "every price tag is accurate", Zhang said. 

She added: "We have required all Wal-Mart stores in China to conduct self-investigation," noting Chinese New Year promotions involve tens of thousands of products. She promised Wal-Mart would improve price tag management. 

"We also welcome the supervision from all the consumers and the government departments."