A charity campaigning against modern-day slavery has petitioned US Customs and Border Protection to ban clothing imports from Boohoo Group Plc and most factories in Leicester, England.

Liberty Shared, based in Washington, D.C., said it submitted two petitions at the beginning of February asking for an investigation as the concentration of suppliers in Leicester, the main UK hub for clothing manufacturing, means “products sold by Boohoo are potentially produced at least in part by forced labour.”

The US bans many products believed to be produced with forced labour, including diamonds mined in Zimbabwe and furniture made in Mexican jails.

Shares in Boohoo, an online fast-fashion retailer, fell more than 7 percent in London in early trading.

The company has not been notified of any investigations by US customs officials, a spokesman for Boohoo said in an emailed statement. The retailer is confident that all products meet or exceed the agency’s criteria on “preventing the product of forced labour entering the US or any of our markets,” he said.

UK enforcement bodies have found no evidence of modern-day slavery after months of investigations, the spokesman said.

From March through August, sales in the US nearly doubled year-on-year, reaching £202 million ($280 million), Boohoo said in September.

NastyGal, PrettyLittleThing

The company, which owns the brands NastyGal and PrettyLittleThing, is known for producing catwalk-inspired items within a few weeks, selling them only online. It sources 40 percent of its garments from the UK, mostly from Leicester, rather than from overseas, in order to process faster orders.

The retailer’s supply chain has been under intense focus in the past year since allegations emerged that workers at supplier factories in Leicester were paid less than minimum wage and were working in unsafe conditions. Boohoo commissioned Alison Levitt, a prominent UK lawyer, to carry out an independent review, which found that Boohoo had prioritised profit and growth and ignored concerns about labour violations.

The review cleared the company of direct involvement. Boohoo has appointed UK judge Brian Leveson to supervise an overhaul of its processes.

By Lisa Pham and Deirdre Hipwell