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."
Another ex-prisoner who now lives in the UK told Humphrey that prisoners had been packaging Christmas cards and gift tags for Tesco for at least two years. The reporter himself, who said he was jailed on "bogus charges" for his work as a corporate fraud investigator, said that during his time in Qingpu prison, he had seen "product tags with the names of other high street brands."
"We abhor the use of prison labor and would never allow it in our supply chain," a spokesperson for Tesco said in a statement.
"We were shocked by these allegations and immediately suspended the factory where these cards are produced and launched an investigation. We have also withdrawn these cards from sale whilst we investigate," they added.
"We have a comprehensive auditing system in place and this supplier was independently audited as recently as last month and no evidence was found to suggest they had broken our rule banning the use of prison labor," the company said, adding that if a supplier breaches the rules, they would be immediately and permanently de-listed.
CNN has tried to reach Chinese authorities for comment on allegations of forced labor at Qinpu but was unable to reach anyone.
The supermarket donates £300,000 ($390,000) a year to charities including the British Heart Foundation, Cancer Research UK and Diabetes UK from sales of the cards.