British grocery chain Tesco has suspended a Chinese supplier of its charity Christmas cards after a customer found a message in one of the cards saying they were produced using forced labour.

Tesco says it has 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 labour practices.

It emerged late on Monday that the printing company was also a supplier to Australia’s Cotton On Group.

The newspaper said the potentially problematic provenance of the cards came to light when a six-year-old girl in south London found a card in her box already had a message written inside.

It read: “We are foreign prisoners in Shanghai Qinqpu prison China forced to work against our will. Please help us and notify human rights organisation.”

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.

Six-year-old Florence Widdicombe said she was “shocked” to see the message in the card.

“We opened them about a week ago and we were writing in them, and on about my sixth or eighth card, somebody had already written in it,” she told BBC TV.

Her father, Ben Widdicombe, said on Sunday they at first thought the message was a prank.

“On reflection, we realised it was actually potentially quite a serious thing,” Mr Widdicombe said.

“I felt very shocked, but also a responsibility to pass it on to Peter Humphrey, as the author asked me to do.”

He 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.”

Mr Widdicombe said finding the message shortly before Christmas was poignant and that he 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.

Mr Humphrey and his American wife, Yu Yingzeng, were jailed in China in 2014 for illegally obtaining private records of Chinese citizens and selling the information to clients, including drugmaker GlaxoSmithKline.

They were deported from China in June 2015 after their jail terms were reduced.

Mr Humphrey told the BBC he thought he knew who wrote the message. but would not identify the person for fear they might face retribution.

Mr Humphrey said he was “pretty sure” the message 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 labour in its supply chain.

“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,” a spokesman said.

“If a supplier breaches these rules, we will immediately and permanently delist them.”

Tesco is Britain’s biggest retailer, donates £300,000 ($565,412) a year from the sale of charity Christmas cards to the British Heart Foundation, Cancer Research UK and Diabetes UK.

-with AAP