The Business Secretary, Vince Cable, must force British clothing companies to investigate whether garments sourced from Cambodia are made at factories with fair labour practices, the unions have warned.

Frances O'Grady, general secretary of the Trade Union Congress, has written to Mr Cable warning that "repression against workers taking strike action" is growing in Cambodian factories which are increasingly supplying the UK and Europe's demand for cheap clothing and footwear. The country's exports to the EU were worth nearly £1.1bn in the first 11 months of last year, up 31 per cent on the same period in 2012.

In January, H&M, Gap, Puma and Adidas were among a number of Western clothing groups that wrote to Hun Sen, the Cambodian Prime Minister, demanding that his government stop using violence against striking workers.

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Five garment workers had just been shot dead while striking for higher wages, including one who was employed by a supplier to Puma.

"European-based corporations have a responsibility to ensure that workers' rights are respected in their supply chains," said Ms O'Grady's letter, which was also sent to the Foreign Secretary, Philip Hammond, as the House of Commons broke for summer recess on 22 July.

Cambodia benefits from a preferential EU trading regime for particularly poor countries. However, a condition is that beneficiaries must guarantee fundamental labour rights, both in law and in practice.

Last year, an International Labour Organisation report said that working conditions in Cambodian garment factories had worsened after a steady improvement in between 2005 and 2011. Child labour and fire safety were highlighted as two areas of concern.

The International Trade Union Confederation and its European equivalent want the EU to negotiate what is known as a "Sustainability Compact" with Cambodia. Modelled on an agreement signed with Bangladesh last year, this would build on current trade conditions and force Cambodia to create a system that would allow trade unions and collective bargaining.

"If the labour provisions of trade preference schemes are to be a tool for raising labour standards and combating income inequality, the EU must take enforcement seriously," reads the letter. "Additionally, the TUC urges the Government to raise this matter in its bilateral relations with Cambodia, and with all... businesses in the UK sourcing garments from factories in Cambodia." Ms O'Grady has not yet had a reply.