Supermarket giant Tesco said Sunday it has stopped production at a factory in China after one of its Christmas cards was found to contain a cry for help from a prisoner who made it.The Sunday Times newspaper reported that a girl in south London had opened a card last weekend to find a message inside claiming to be from inmates at Shanghai's Qingpu Prison."We are foreign prisoners in Shanghai Qingpu Prison China," said the message, in a Tesco charity card featuring a kitten in a Santa hat."Forced to work against our will. Please help us and notify human rights organisation."A spokeswoman for Tesco, Britain's biggest retailer, said it was "shocked" at the news and had "immediately halted production at the factory where these cards are produced". "We would never allow prison labour in our supply chain," she said, adding that an investigation had now been launched.She said the company had a "comprehensive auditing system in place"."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," she said."If evidence is found we will permanently de-list the supplier."According to the Sunday Times, the note in the card - sold to raise money for charity - asked whoever received it to contact "Mr Peter Humphrey". The girl's father searched for Humphrey online and discovered that he was a former journalist who had spent two years in Qingpu.He got in touch with Humphrey, who contacted some other ex-prisoners, who confirmed that foreign inmates had been backing cards for Tesco.Humphrey then wrote up the story for The Times.Humphrey and his wife Yu Yingzeng, a naturalised US citizen, ran an investigative firm hired by pharmaceutical giant GlaxoSmithKline (GSK).Arrested in 2013 on suspicion of illegally obtaining private data, he confessed on Chinese state TV but later claimed he was forced to.He and his wife were convicted in August 2014 and deported the following June.