The report, by charity Oxfam and workers' rights group the TUC, says staff are being forced to put in longer hours for less money.

They found that workers in Bulgaria, Cambodia, Thailand, China, Indonesia and Turkey were being poorly treated.

The Olympic Games are taking place in Athens, Greece, in the summer.

A Thai worker told Oxfam: "We work until 2am or 3am. We always have to work a double shift."

Respect

An Oxfam spokesman said: "We need to change the way these companies do business to make trade fair."