Primark is conducting an investigation into one of its key suppliers after an undercover probe found illegal immigrants were being employed at just over half the minimum wage.

The value fashion chain said it would investigate Manchester-based supplier TNS Knitwear after The Observer and the BBC conducted an undercover investigation into practices at the garment firm.

TNS Knitwear supplies 20,000 garments a week to Primark and workers were caught on camera admitting that they were working there illegally. Some were allegedly being paid as little as£3 an hour.

Another supplier to Primark, Fashion Waves, was investigated and found also to have illegal workers employed.

Work conditions in both factories are claimed to be poor and in breach of health and safety regulations.

A spokesman for Primark said: "Primark was informed this week that one of the UK factories from which it buys some knitwear has allegedly broken a number of UK employment, tax and immigration laws.

"Primark is conducting its own investigation. Once that investigation is completed, it will decide how to proceed. Meanwhile, Primark has handed all relevant information passed to the company over to the relevant enforcement agencies so they may take action.”

He added: "We are extremely concerned about the very serious allegations made against our supplier TNS Knitwear and against TNS's unauthorised subcontractor, Fashion Waves."