After a 6-year-old girl found a plea for help from Chinese prison laborers inside a Christmas card she was preparing to send to friends, the U.K. grocery chain Tesco said it halted production at the factory in China where the cards were produced.

Tesco said it also launched an investigation of the Chinese supplier it hired to make the holiday cards, Zheijiang Yunguang Printing, after the Sunday Times raised questions about the factory's labor practices. The newspaper reported the factory used forced labor to produce charity Christmas cards for the supermarket.

The potentially problematic source of the cards came to light when Florence Widdicombe, 6, of London was writing holiday cards to friends and found one that already had a message written inside. The statement read: "We are foreign prisoners in Shanghai Qinqpu prison China forced to work against our will. Please help us and notify human rights organization."

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The writer asked whoever received the note to contact Peter Humphrey, a former British journalist who was detained in China while working as a corporate investigator and spent time at the same Shanghai prison.

"I felt very shocked, but I also felt the responsibility to pass it on to Peter Humphrey as the authors asked me to do," the girl's father, Ben Widdicombe, said. He added that at first they believed the message was a "prank."

"Felt really sad"

Widdicombe said finding the message shortly before Christmas was poignant and that he told his daughter, Florence, what it might mean.

"We explained that the person who wrote it was a prisoner in China and that the person felt the prison guards were being mean, making them do work, they felt really sad," the father said.

Humphrey told the BBC he thinks he knows who wrote the message.

"I spent two years in captivity in Shanghai between 2013 and 2015 and my final nine months of that captivity was in this very prison, in this very cell block where this message has come from," he said. "So, this was written by some of my cellmates from that period who are still there serving sentences."

He said he won't identify the person for fear the inmate would face retribution. He added he was "pretty sure" it was put inside the card by a group of prisoners as a collective request for help.

Tesco said it was "shocked" by the discovery and would never allow prison labor in its supply chain.

"We abhor the use of prison labour and would never allow it in our supply chain," the company 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."

The company says it donates 300,000 pounds ($390,000) annually to the British Heart Foundation, Cancer Research UK and Diabetes UK from the sale of its Christmas cards.

The girl's father said the message was eye opening: "It hits home there are injustices in the world and difficult situations that we know about and read about each and every day."