British supermarket chain Tesco has suspended production at a factory in China after a letter from alleged slave labourers was discovered in a box of charity Christmas cards.

Tesco has come under fire after a six-year-old girl in London, Florence Widdicombe, discovered a message from Chinese prisoners inside a Christmas card.

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

The discovery of the letter led Tesco to suspend production at its Zheijiang Yunguang Printing factory and say that it will de-list the factory if the forced labour allegations are true.

“We were shocked by these allegations and immediately halted production at the factory where these cards are produced and launched an investigation”, a Tesco spokesperson told the BBC.

Tesco claims to have inspected the facility last month, claiming that there was no evidence found of forced labour during the inspection.

The letter from inside the prison went on to ask the person who discovered the message to contact Mr Peter Humphrey, a British reporter, who spent nearly two years locked up in Chinese prisons on “bogus charges”.

“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 from the suburban prison where I spent nine of my 23 months”, Humphrey wrote in The Times.

“I got to know many foreign inmates in Qingpu, and for a while found ways to contact them after my release. But prison sources told me that censorship of outbound and inbound correspondence has been tightened up this year, which may be why none of the prisoners I know was able to write to me about their work directly. So they resorted to the Qingpu equivalent of a message in a bottle, scribbled on a Tesco Christmas card”, he concluded.

Mr Humphrey added that he contacted several former inmates of the Qingpu prison, some of whom confirmed that inmates are being forced to do unpaid manual labour, including packing Christmas cards for Tesco.

Forced labour in China has been commonplace since 1949 when the communists came to power under Mao Zedong, who introduced the laogai “reform through labour” system — sometimes referred to as the “bamboo gulag” — which is believed to be responsible for tens of millions of deaths.

Though the laogai system was technically abolished in 1994, the structure of the system remains largely in place to this day, according to the Laogai Research Foundation.

“Prisoners, who are typically unpaid, provide a free source of labour in prison-run factories, farms, workshops, and mines, enabling these “businesses” to reap huge profits”, according to the foundation.

“Unfortunately, due to intentional deception on the part of the Chinese government, lax international labelling requirements, and reliance on middlemen exporters, Laogai products are difficult to identify and continue to find their way onto store shelves worldwide,” the foundation explained.

A similar note to the one found in the Tesco Christmas card was found in a pair of Primark trousers in Belfast in 2014. The letter translated into English read:

“SOS! SOS! SOS!

We are prisoners in the Xiang Nan Prison of the Hubei Province in China. 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. We work as hard as oxen in the field.

“We call on the international community to condemn the Chinese government for the violation of our human rights!”

Follow Kurt on Twitter at @KurtZindulka