The British supermarket chain Tesco has suspended production at a Chinese factory after a six-year-old found a note in a box of Christmas cards alleging to be from a prisoner.
The girl from London discovered the message as she wrote cards for her friends.
It claimed foreign inmates were being forced to work against their will.
Six-year-old Florence Widdicombe told Sky News "it was really weird" to find the note in the charity card.
The message claimed: "We are foreign prisoners in Shanghai Qingpu prison China.
"Forced to work against our will. Please help us and notify human rights organisation."
In a statement, Tesco UK said: "We abhor the use of prison labour and would never allow it in our supply chain.
"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 We have also withdrawn these cards from sale whilst we investigate."
The company 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 labour.
"If a supplier breaches these rules, we will immediately and permanently de-list them."
The note found in the cards is reported to have contained a note to contact Peter Humphrey - who it transpired is a former British journalist who had spent two years in jail in China, including time at Qingpu.
Florence's father contacted the journalist via LinkedIn.
Writing in The Sunday Times, Mr Humphrey said: "I do not know the identities or nationalities of the prisoners who sneaked this note into the Tesco cards, but I have no doubt they are Qingpu prisoners who knew me before my release in June 2015".
