London (CNN) British supermarket Tesco says it has suspended production at a Chinese factory following allegations that prisoners were forced to pack charity Christmas cards.

The Sunday Times reported that six-year-old Florence Widdicombe, who lives in Mitcham, south London, opened a box of Tesco charity Christmas cards and discovered a desperate message inside one of them.

"We are foreign prisoners in Shanghai Qingpu prison China," the message read. "Forced to work against our will. Please help us and notify human rights organization."

The card also urged the recipient to "contact Mr Peter Humphrey" -- a former British journalist who happened to have spent two years in Chinese prisons, including Qingpu. Widdicombe's father contacted Humphrey, who reported the story for The Sunday Times.

Humphrey wrote that he contacted several members of an "informal network of ex-prisoners," who told him that inmates in the foreign prisoners unit are being forced into "mundane manual assembly or packaging tasks."

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