Tesco has shut down production at a factory in China alleged to have used forced labour to make charity Christmas cards, after a south London schoolgirl discovered a desperate message from a “foreign prisoner” inside her card.

Florence Widdicombe, a six-year-old girl from Tooting, opened a card with a kitten wearing a Santa hat and found a note explaining that inmates were being “forced to work against our will”.

The message stated: “We are foreign prisoners in Shanghai Qingpu prison China. Forced to work against our will. Please help us and notify human rights organisation,” according to The Sunday Times.

Florence, who was writing cards to school friends last weekend when she found the note, told reporters: “We didn’t open [the cards] on the day that we got them, we opened them about a week ago.

“We were writing in them. About on my sixth or eighth card, somebody had already written in it.”

The family who found a message from a ‘prisoner’ in a Christmas card said they thought it was a prank when they first read it (PA)

Tesco donates £300,000 each year from the sale of cards made in China to the British Heart Foundation, Cancer Research UK and Diabetes UK.

A Tesco spokesperson said the company was “shocked” by the discovery and had shut down operations at the facility. “We would never allow prison labour in our supply chain,” she said.

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She added: “We were shocked by these allegations and immediately halted production at the factory where these cards are produced, and launched an investigation.”

The message came to light after the schoolgirl’s father Ben Widdicombe contacted Peter Humphrey, a former journalist who was imprisoned in the same Qingpu facility between 2013 and 2015 after being accused of breaking China’s privacy laws.

Mr Humphrey was named in the short message as a potential point of contact along with human rights organisations.

The front of a Tesco Christmas card from the same pack as one found to contain the message (PA)

“When I looked at the message I thought it was incredible and wondered if it was a prank,” said Mr Widdicombe, who said he later realised it would be “wrong not to pass it on” to its intended recipient.

Mr Humphrey said other former inmates at a foreign prison unit at Qingpu told him the packing of Christmas cards had been happening for “at least two years”.

Tesco said the card with the message on it had been made at Zhejiang Yunguang Printing, where an independent audit was conducted in November which found no concerns about forced labour.

The Zhejiang Yunguang Printing operation is reportedly close to the Qingpu prison on the outskirts of Shanghai.

“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,” said a Tesco spokesperson. “If evidence is found we will permanently delist the supplier.”

The supermarket giant said it had not received any previous complaints from customers about messages inside cards.

Jeremy Lune, chief executive of Cards for Good Causes, an organisation providing cards for several charities, said it was “truly shocking” and called for a wider investigation into prison labour used in production.