Wal-Mart audits have detected possible irregularities by Yue Wing Cheong Light Products and Mainland Headwear Holdings, said Jonathan Dong, spokesman for Wal-Mart China.

The companies, both based in the southern manufacturing centre of Shenzhen, have been ordered to issue a report on the suspected violations within 120 days and make any necessary changes, he added.

The companies' books showed discrepancies between paper records of worker attendance and computerised versions, as well as possible overtime violations, he said.

The discrepancies could be due to either sloppy record-keeping or the deliberate employment of workers off the official books.

"But we didn't find underage labor violations at either factory," Dong added.

The companies were among four accused in June of a range of labour violations in the production of official merchandise for the 2008 Beijing Olympics.