The Briton who made public a message in a Christmas card claiming to be from Chinese prisoners involved in forced labour on Monday dismissed Beijing's denial as "lies".
Peter Humphrey, a former fraud investigator and journalist, wrote an article about the note allegedly penned by foreign inmates in Shanghai's Qingpu prison -- where he himself was once held.
A spokesman for the Chinese foreign ministry denied there was any forced labour by foreign convicts at Qingpu and attacked Humphrey for inventing a "farce" to "hype himself up".
Contacted by AFP for his response, Humphrey -- who is now based in Britain -- said: "It's the kind of answer they have given to every allegation of human rights abuses that is ever mentioned.
"This is really completely to be expected, because nothing except lies ever comes back to the world when any such issue arises," he said.
The note was found by a London schoolgirl in a Christmas card sold by supermarket giant Tesco, and claimed to be from foreign prisoners in Qingpu "forced to work against our will".
Tesco expressed shock at the revelation and announced it was both stopping the sale of the cards and ceasing production at the Chinese factory involved while it investigated.
- 'I know the handwriting' -
The note asked whoever received it to contact Humphrey, who spent nine months in Qingpu during almost two years in custody in China for illegally obtaining personal information -- charges he dismissed as "bogus".
He told AFP he had never met the girl or her family, but when they got in touch and showed him a copy of the note, "I absolutely knew it was true, in my gut, because I know the handwriting."
He would not name the author for fear of repercussions, but noted it was not the first time prisoners in China had got a message out this way.
"It's too dangerous for them to use correspondence, phone calls or consular meetings" to raise concerns about conditions, he said.
Humphrey said he did not hold Tesco responsible, if it were found to have used prison labour.
"China makes it impossible for a company to drill down right to the bottom of the supply chain to identify the small contractors," he said.
And he said he believed the prisoners involved were working against their will.
"They don't mean that they are chained to a factory table and whipped. What they mean is that they have been put in a position where they are coerced," he said.
Humphrey said he had heard from former prisoners that in the last 12-18 months, the jail had blocked remittances from families for inmates' daily necessities like toiletries.
"They blocked that as a way to try and force the prisoners to work in the labour production, where they can earn £10 to £12 (about 12 to 14 euros, $13 to $15) worth of renminbi per month," he said.
He added that a merit system allowing prisoners to shave time off their sentences, for example by working or studying, was now open only to those prepared to do labour.