Tesco says it has suspended production at a factory in China alleged to have forced foreign prisoners to help make charity Christmas cards and also withdrawn them from sale.

The allegations came to light after the Sunday Times reported that Florence Widdicombe, aged six, from Tooting, south London, opened a box of charity Christmas cards from the supermarket and discovered a plea for help inside one of them.

The message read: “We are foreign prisoners in Shanghai Qinqpu prison China. Forced to work against our will. Please help us and notify human rights organization.”

It also urged the reader of the message to contact Peter Humphrey, a former journalist who spent 23 months imprisoned at the same Qingpu prison. Florence’s father, Ben Widdicombe, contacted Humphrey, who took the story to the Sunday Times.

The message from a prisoner in China appeared in one of Tesco’s Christmas cards. Photograph: tesco

The journalist said he contacted fellow ex-prisoners, one of whom said that for at least two years inmates in the foreign prisoner unit had been picking designs and then packaging the Tesco cards, sealing the boxes and putting them into shipping cartons. He said they were also involved in packing Tesco gift tags.

Humphrey, who was released from prison in June 2015, said that during his time in Qingpu he saw product tags with the names of other high street brands.

Florence said she had already written in several other cards when she discovered the message. “It was a bit funny and I felt a bit shocked,” she said.

Her father said he felt “incredulity” when he read the message and initially thought it was a prank. “On reflection we realised it was actually potentially quite a serious thing, so I felt very shocked, but also a responsibility to pass it on to Peter Humphrey as the author asked me to do,” said Florence.

A spokesman for Tesco 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 We have also withdrawn these cards from sale while we investigate.

“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.”

Similar notes from Chinese prisoners have been reported in the past. In 2017, Jessica Rigby, from Braintree, Essex, said she found a handwritten note in a Christmas card contained within a box bought from Sainsbury’s. It translated as: “Wishing you luck and happiness. Third Product Shop, Guangzhou Prison, No 6 District.”

In 2014, Karen Wisínska from Co Fermanagh, Northern Ireland, said she found a note in a pair of Primark trousers she bought in Belfast alleging slave labour conditions in a Chinese prison making clothes for export. The author, who claimed to be an inmate of the Xiang Nan prison in Hubei province, said: “Our job inside the prison is to produce fashion clothes for export. We work 15 hours per day and the food we eat wouldn’t even be given to dogs or pigs.”

Amnesty International says forced labour persists in Chinese detention facilities.

Tesco donates £300,000 a year to three charities – the British Heart Foundation, Cancer Research UK and Diabetes UK – from the sale of boxes of Christmas cards and it is understood they will still receive the full donation this year.

Humphrey and his wife, Yu Yingzeng, a naturalised US citizen, were detained in 2013. They had become embroiled in the fallout from a GlaxoSmithKline corruption scandal that their company was originally hired to investigate. He previously told the Guardian that at times during his imprisonment he was handcuffed and held in a metal cage.

Humphrey was sentenced to two and a half years in jail and fined 200,000 yuan (£20,260). His wife was sentenced to two years and fined 150,000 yuan. They were released early on medical grounds.