Fashion brand Gap has revealed that many of its garments are produced in sweatshops worldwide.

The San Francisco-based retailer said the discovery came after a 90-strong team visited 3,000 locations where its clothes are produced.

Factories in China, Taiwan and Saipan were the worst offenders. Many workers were subject to psychological or verbal abuse while some of the workhouses were fire-traps. There were few instances of workers being physically maltreated.

Gap says that 136 factories have been removed from its approved contractors list.

"We believe that garment and other manufacturing workers around the world deserve better than the reality that many unfortunately face," said Gap chief executive Paul Pressler.

The report comes amid a shift in the tone of the debate over sweatshops.

High-profile legal battles involving Gap, Nike, J.C. Penney, Levi Strauss & Co., Tommy Hilfiger and other companies have boosted shoppers' awareness of worldwide labour conditions.

That, in turn, has increased pressure on companies to disclose where clothes are made, what workers are paid and whether factory managers treat them humanely.

Data sourced from: CBS Marketwatch; additional content by WARC staff