One of Britain's most popular fashion chains is under pressure to sever its links with clothing suppliers that buy cotton from Uzbekistan, where large quantities are harvested using child labour.

H&M, the giant fashion chain which uses football star David Beckham and singer Lana Del Rey in its advertising campaigns, has signed a pledge to "not knowingly" source cotton from the central Asian country in response to concern from human rights groups.

Uzbekistan, one of the world's largest exporters of cotton, forces adults and children as young as nine to pick cotton under what the charity Anti-Slavery International describes as "appalling conditions". However, it is difficult to trace Uzbek cotton back to its source. Much of it ends up in Bangladesh and China. As a result, many fashion chains cannot guarantee that their clothing is free of Uzbek cotton.

H&M, which has almost 200 UK stores and 2,500 worldwide, is now one of several high street names that will be targeted by Anti-Slavery International's cotton campaign, which calls on well-known brands to stop buying clothing from suppliers that source cotton from Uzbekistan. "Unless H&M implements the practical steps set out by the cotton campaign, it is near impossible for us to be confident that H&M's goods are truly free from state-sponsored forced labour," a spokeswoman for the charity said. "By implementing these steps, we can be confident that H&M is doing everything it can to support an end to the use of forced labour."

It is believed that the Uzbekistan government increased the scale of forced labour for this year's harvest, which was worth an estimated $1bn (£619m). Schoolchildren and their teachers, doctors and nurses and many other public sector employees were forced to work in the cotton fields or face fines, the loss of wages or denial of college places. International monitors were banned from observing the harvest.

Human rights groups said students were beaten by public authorities for failing to meet their quotas. In Angren district, parents were told that if they wanted to exempt their children from the harvest they would either have to pay a $110 fee or withdraw their children's application to high school. All government employees are eligible to be sent to pick cotton. A refusal can mean the loss of a job, pay or benefits.

An H&M spokeswoman said it had a policy of prohibiting the use of Uzbek cotton in its products, which is communicated to all suppliers. "While our systems continue to evolve, we aim for all cotton to come from more sustainable, fully traceable sources by 2020 at the latest."

Ikea has been subjected to similar scrutiny from human rights groups. "We do all we can to ensure that Uzbek cotton is not used in our products, but the traceability process for the cotton industry is not robust," said Josefin Thorell, an Ikea spokeswoman. "In the case of, for example, Bangladesh, where much of the cotton used in manufacturing is traditionally from Uzbekistan, Ikea suppliers are required to buy from India."

Concerns about the use of Uzbek cotton have led to questions being asked of Peter Lilley, the former Tory trade secretary who heads the Uzbek-British Trade and Industry Council, which promotes the Uzbek Cotton and Textile Fair. Anti-Slavery International questioned whether the council was in breach of UN and Foreign Office human rights guidelines. In an emailed reply, Lilley said the council followed Foreign Office guidelines and its main role was "to broaden trade and investment between the UK and Uzbekistan", not promote cotton.