Three big Western companies are investigating allegations that prisoners in China made packaging bearing their brand names.

Household products group 3M, and fashion retailers H&M and C&A are responding to an article published by the Financial Times last week in which a British corporate investigator who was imprisoned in China said he witnessed fellow inmates making packaging that carried the names of the companies.

Peter Humphrey was arrested in China in 2013 on charges that his risk consultancy firm illegally acquired private data of Chinese citizens while doing work for corporate clients. He has consistently denied the charges, but was found guilty in 2014 and spent nearly two years behind bars.

"Our men made packaging parts. So much for corporate social responsibility," Humphrey wrote in the Financial Times article, referring to his fellow prisoners. "I recognized well-known brands -- 3M, C&A, H&M."

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Humphrey wrote that the firms involved may not have known prison labor was being used to make their products. The inmates earned about 120 yuan ($19) a month for their work, he added. That's a fraction of average wages in China's vast manufacturing industry.

3M (MMM), which is best known for making Scotch Tape and Post It notes, said it "does not engage or participate in exploitative working conditions, and we are not aware of any 3M suppliers in China using prison labor."

"We take our commitment seriously and are investigating Mr. Humphrey's report," the Minnesota-based company said in a statement.

H&M (HNNMY) said that it couldn't confirm whether Humphrey's allegations were true, but added that it was taking them "very seriously."

"We are currently looking for further and more detailed information together with our China-based team in order to make an investigation," it said, adding that prison labor "seriously violates the regulatory framework that our suppliers must follow."

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C&A said it was not aware of the use of prison labor in its Chinese supply chain but would "try to obtain more information on the case to be able to conduct a formal investigation."

It added that it has processes in place to identify subcontracting practices that could lead to forced labor.

Humphrey didn't specify in his article whether the prisoners were being forced to work on the packaging bearing the brand names.

China's Ministry of Justice didn't respond to requests for comment on the subject of prison labor. The prison service in Shanghai, where Humphrey was jailed, declined to comment.

The US government has in recent years stepped up efforts to battle forced labor in prisons in China and other countries.

A report last year by the US-China Economic and Security Review Commission, a group set up by the US Congress to monitor relations between the two countries, said China still operates a network of prisons that use forced labor to make goods for export.

Here are the full statements from the companies mentioned in Humphreys' report:

3M

"3M is committed to responsible and sustainable operations, and selects suppliers that share our commitment. As specified in our Supplier Responsibility Code, 3M expects its suppliers to be in full compliance with all applicable labor and human resource laws, as well as committed to upholding the human rights of workers and treating them with dignity and respect as understood by the international community.

3M does not engage or participate in exploitative working conditions, and we are not aware of any 3M suppliers in China using prison labor. We take our commitment seriously and are investigating Mr. Humphrey's report."

H&M

"It is completely unacceptable placing manufacturing into prisons and it seriously violates the regulatory framework that our suppliers must follow. Since it is a non-negotiable requirement, a failure to comply would immediately lead to permanent termination of our business contract.

We are aware of the claims and we take it very seriously. At this point we can't confirm whether they are accurate or not. We are currently looking for further and more detailed information together with our China based team in order to make an investigation."

C&A

"We never tolerate any kind of forced or bonded labor in our supply chain. This is backed by our rigorous supplier code of conduct and audit program, where if forced labor is found, the supplier is immediately terminated. This includes any kind of prison labor. We audit all 273 of our suppliers' factories in China at least annually, and have not observed or been made aware of the use of prison labor in our Chinese supply chain. To enhance our approach, our sustainable supply chain auditors and quality assurance teams have practices in place to detect unauthorized subcontracting that could ultimately lead to forced or bonded labor at the factory level.

We take these allegations mentioned by the author very seriously and will try to obtain more information on the case to be able to conduct a formal investigation."

-- Nanlin Fang contributed to this report.