The company said there was no proof convicts had been involved, but stringent action was being taken to protect the Adidas name.

All the current licensing contracts for ball manufacturing in China have been cancelled while the factories are being investigated. Adidas have also cancelled contracts with a Japanese sub-contractor.

The investigation is difficult because of the high proportion of counterfeit footballs being produced bearing the Adidas logo. The company has raided at least one factory in China but found nothing.

Factories to be reviewed

A spokesman for Adidas, Peter Csanadi, said the company was reviewing all its factories involved in the manufacture of footballs for sale to the public, but warned that shutting factories on the strength of allegations of wrongdoing could lead to the loss of genuine jobs.

Mr Csanadi said that from now on football production would be controlled through an Adidas subsidiary in Hong Kong, and third parties would not be involved.

Adidas is one of the 11 official sponsors of the World Cup and supplies the match balls for the entire tournament. These balls are not the subject of any investigation.