According to a report in the Sunday Times, Florence Widdicombe, from Tooting, south London, discovered that one of the cards in her box from the retailer had already been used.

The card, which was part of a box that cost £1.50 and featured a kitten in a Santa hat, contained a handwritten message from a Chinese gulag that read, in capital letters: “We are foreign prisoners in Shanghai Qingpu prison China. Forced to work against our will. Please help us and notify human rights organisation.”

The card also urged the finder to contact journalist Peter Humphrey, without explaining why. Florence Widdicombe's father Ben Widdicombe did so and Humphrey took the story to the Sunday Times, which published his report yesterday (22 December).

Humphrey said his past work in Shanghai as a corporate fraud investigator had “upset the Chinese government, which jailed both me and my American wife Yu Yingzeng, on bogus charges that were never heard in court”.

He added he had “no doubt” that those responsible for sneaking the note into the Tesco card are Qingpu prisoners who knew him before his release from the prison in June 2015.

A spokesperson for Tesco said: “We would never allow prison labour in our supply chain. We were shocked by these allegations and immediately halted production at the factory where these cards are produced and launched an investigation.

“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 evidence is found we will permanently de-list the supplier.”

The Chinese factory involved in the printing of the cards, Zhejiang Yunguang Printing, told China's Global Times today, "we have never done such a thing" and said it only became aware of the situation when it was contacted by foreign media. The company's factory manager told the BBC today that it had not outsourced any of its work to the Qingpu prison.

In his Sunday Times report, Humphrey wrote that the incident has highlighted the difficulty of monitoring the production of cheap goods in China, “where sub-contracting is common and the use of forced labour is often difficult to detect”.

He added it would be difficult for retailers to locate supply chain issues.

“The daunting reality is that China’s prisons are closed to independent auditors who have little chance of unravelling the secretive business arrangements that have turned the jail system into a lucrative profit centre for the Chinese state.”

Tesco donates £300,000 a year from the sale of the Christmas cards to the British Heart Foundation, Cancer Research UK and Diabetes UK.

In a joint statement, the three charities said: “Like Tesco, we’re shocked by these allegations. We are in touch with Tesco, who have assured us that these particular cards have been removed from sale, and that the factory producing them has been suspended while they investigate further. We await the outcome of Tesco’s full investigation.”

Speaking to BBC Radio 4’s Today programme this morning, Peter McAllister, director of the Ethical Trading Initiative (ETI), of which Tesco is a founding member, said: “As we see supply chains go into many different tiers and across many different countries, we do find still that some people are prepared to abuse and exploit workers.

“In this case it would appear that somehow, deep in the supply chain, that has got mixed up with use of prison labour.”

The first clause of the ETI's Base Code says that 'employment is freely chosen' and Today presenter Martha Kearney questioned, when we see cases like this, whether the code is still meaningful.

McAllister replied: “If we don't have standards that are set against the global conventions that have been agreed by nations then we don't have a reference point. I think the key thing that we're looking at is how do you implement these standards in these globalised conditions in various different countries which may have good laws or good policies in place.

“Tesco have taken the decision to suspend that factory; they're going to do an investigation and we'll want to learn from them what's worked, what's not worked, and how have they found this situation in their supply chain.”

He added: “I don't think the fail is with the code. I think we recognise at ETI amongst all the members that we work very much in the real world. Whether that is modern slavery in the UK, this situation in China, or something else in a different place, the code serves a purpose because it's a reference to say what is happening and how do we drive out these changes.

“We recognise, and responsible companies recognise, that you can never be 100% sure and you need to be vigilant consistently if you're trying to eliminate these practices.”

Seth Woodmansterne, deputy managing director at Woodmansterne Publications, which prints Christmas cards for a range of charities, told Printweek: “At Woodmansterne we manufacture all our products in the UK: from the design, print and finish, through to the packing and distribution.

“This helps ensure a stable, efficient and reliable supply chain crucial to ensuring our exacting ethical standard, as well as reducing our carbon footprint from transportation.”