Fast fashion retailer Forever 21 is being investigated by the US Department of Labor (DoL) for "significant" labour law violations among the contractors and manufacturers who supply its apparel.

The Department of Labor said it had recently found evidence of significant violations of the Fair Labor Standards Act's minimum wage, overtime and record-keeping provisions by vendors in Southern California supplying goods to Forever 21.

But Forever 21 has refused to comply with a subpoena to provide back-up documents, and steps are now being taken to force the retailer to comply.

"We are disappointed that Forever 21 has refused to cooperate with the subpoena, particularly given the significant problems we have found among its suppliers," said Ruben Rosalez, regional administrator for the division in the West.

"Since 2008, our investigators have identified dozens of manufacturers producing goods for Forever 21 under sweatshop-like conditions.

"When companies like Forever 21 refuse to comply with subpoenas, they demonstrate a clear disregard for the law, and the Labor Department will use all enforcement tools available to recover workers' wages and hold employers accountable."

The investigation is focusing on employers in the Los Angeles and Orange counties, as well as those operating out of large garment buildings in Los Angeles' Fashion District.

The Department of Labour said that over the past five years its Los Angeles, San Diego and West Covina offices have carried out more than 1,500 investigations and that violations were found in 93% of them.

The Fair Labor Standards Act (FLSA) requires workers to be paid at least the federal minimum wage of $7.25, plus time and one-half their regular rates, including commissions, bonuses and incentive pay, for hours worked beyond 40 per week.