British supermarket giant Tesco suspended a Chinese supplier of Christmas cards on Sunday after it was reported that a customer found a message written inside a card saying it had been packed by foreign prisoners who were victims of forced labour.

"We abhor the use of prison labour and would never allow it in our supply chain," a Tesco spokesperson said on Sunday.

"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 We have also withdrawn these cards from sale whilst we investigate."

Tesco, Britain's biggest retailer, donates £300,000 ($520,000 Cdn) a year from the sale of the cards to the charities British Heart Foundation, Cancer Research UK and Diabetes UK.

The message inside the card was found by six-year-old Florence Widdicombe in London. (Reuters )

The Sunday Times said the message inside the card read: "We are foreign prisoners in Shanghai Qingpu Prison China. Forced to work against our will. Please help us and notify human rights organization.

"Use the link to contact Mr Peter Humphrey."

Contact information for British man

Peter Humphrey is a British former journalist and corporate fraud investigator.

Humphrey and his American wife, Yu Yingzeng, were both sentenced in China in 2014 for illegally obtaining private records of Chinese citizens and selling the information to clients including drugmaker GlaxoSmithKline. The couple were deported from China in June 2015 after their jail terms were reduced.

A staff member who answered the company's main telephone line on Monday told Reuters they were unaware of the press reports or Tesco's comments. She declined to provide her name. The company did not respond to further emailed requests for comment.

The message inside the card was found by a six-year-old girl, Florence Widdicombe, in London, the Sunday Times said. Her father contacted Humphrey via the LinkedIn social network.

"We didn't open them on the day that we got them. We opened them about a week ago. We were writing on them, and on my sixth or eighth card somebody had already written in it," Florence told Reuters.

The message written inside a card sold at Tesco. (Reuters)

Florence's father, Ben Widdicombe, said he felt shocked after his child found the note, "but I also felt the responsibility to pass it on to Peter Humphrey as the authors asked me to do."

Writing in the Sunday Times, Humphrey said he did not know the identities or the nationalities of the prisoners who put the note into the card, but he "had no doubt they are Qingpu prisoners who knew me before my release in June 2015 from the suburban prison where I spent 23 months."

Tesco said it had a comprehensive auditing process in place.

Tesco auditing process

"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," the spokesman said.

"If a supplier breaches these rules, we will immediately and permanently de-list them."

The cards were produced at the Zheijiang Yunguang Printing factory, which is about 100 kilometres from Shanghai Qingpu prison, Tesco said.

Watch: Ex-prisoner recalls receiving holiday card plea

Peter Humphrey, who spent time in a Chinese prison, recalls receiving a Christmas card from fellow detainees pleading for help. 1:17

The company, which prints cards and books for food and pharmaceutical companies, says on its website it supplies Tesco.

Two phone calls and one emailed request for comment to the company went unanswered after usual business hours on Sunday.

Humphrey and his wife said in their trial they had not thought they were doing anything illegal in their activities in China.