A six-year-old girl who found a message from a prisoner in China inside a Tesco Christmas card has said she "thought it was a prank".

Florence Widdicombe told Sky News "it was really weird" to find the note in the charity card.

The message read: "We are foreign prisoners in Shanghai Qingpu prison China. Forced to work against our will. Please help us and notify human rights organisation."

Tesco said it was shocked by the find and had started an investigation, and has also stopped working with the Chinese factory where the card - decorated with a kitten wearing a Santa hat - was made.

The message inside named the former journalist Peter Humphrey as a contact, who spent two years in the same prison.

Florence, who lives with her family in Tooting, London, said: "I was sitting down at the table writing my cards to my friends when I opened one and started laughing because someone had already written in the card.

"Then I passed it on to my mum. It took an hour to get our heads round it because we thought it was a prank."

Her father Ben Widdicombe said he was unsure of the veracity of the message, but contacted Mr Humphrey.

He told Sky News: "After some reflection we thought that if someone was genuinely desperate enough to write that card we should take it seriously. When I tracked Peter down on the internet, I came across this story and it became very serious and quite chilling in a way."

Mr Humphrey said the message from Mr Widdicombe took him back to the "painful two-year period" when he and his wife were imprisoned over "bogus charges".

Writing in the Sunday Times, Mr Humphrey said he did not know who wrote the message but had "no doubt" they knew him during his time in the prison, which is 62 miles (100km) from the Zheijiang Yunguang Printing factory where the cards are supposed to be made.

He said ex-prisoners had confirmed that inmates in the foreign prisoner unit "are being forced into mundane manual assembly or packaging tasks" - including packing Christmas cards and gift tags for Tesco for at least the past two years.

Mr Humphrey said prisoners also make packaging and tags for Western clothing and that he had seen the names of other high street brands on the tags when he was in the prison.

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

A Tesco spokesman told Sky News: "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."

Mr Humphrey said he did not believe British companies would "knowingly commission prison labour, but they may never be able to tell if their Chinese suppliers are sub-contracting production to the prison system".

He said auditors are not allowed into the prisons so there is "little chance of unravelling the secretive business arrangements that have turned the jail system into a lucrative profit centre for the Chinese state".

A spokesperson for the British Heart Foundation Cancer Research UK and Diabetes UK 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."

In 2017, Jessica Rigby, from Braintree, found a message in a Christmas card from Sainsbury's which read in Chinese: "Wishing you luck and happiness. Third product Shop, Guangzhou Prison, Number 6 District."

And in 2014, a woman in Belfast said she had found a note written in Chinese and wrapped inside a prison identity card inside some trousers from Primark.

The note, which had SOS at the top, claimed inmates at Xiangnan jail in Hubei, China, were forced to work 15-hour days producing clothes.

Primark said it regularly inspected its factories and "no prison or other forced labour of any kind was found during these inspections".