In the letter to G4S CEO Ashley Almanza, MRS CEO Jane Frost said: “I would like to understand under what auspices this activity is being undertaken and what assurances you can give me as to the standards under which you are operating.”

Frost wrote that: “Reputable market research companies and individuals are members of the MRS. As such, they operate under a very strict code of conduct which, inter alia, ensures the privacy and rights of respondents are strictly protected and guaranteed adherence to both law and regulation in the area of data protection and other government requirements.”

A copy of the letter was also sent to the Secretary of State for Justice Chris Grayling.

Frost’s letter was prompted by an article published by the Daily Mail, which reported that prisoners at HMP Oakwood, as well as Drake Hall prison, were earning £20 a week conducting telephone surveys on behalf of insurance companies.

The Mail reported that prisoners are given a script to use, asking them to confirm the names and postcodes of the households they call. Homeowners are also reportedly asked to state the value of their possessions and whether they would like to save money on their home insurance.

This later point raised concerns that the calls were sales calls, carried out under the guise of survey research: a practice known as sugging.

However, a G4S spokesman has disputed the Mail’s report. The spokesman told Research that prisoners did not ask people if they were paying too much insurance, and that prisoners were not engaged in sugging.

The spokesman said: “The prisoners are undertaking market research – consumer lifestyle surveys to be exact – and the information they find out gets sent on to data aggregator companies, not insurance companies specifically.”

In addition, a Ministry of Justice spokesman said: “Prisoners placed in call centres are risk assessed and their work subject to stringent security measures, with calls supervised and recorded.

“At no point can they ask the value of items, record data outside of the secure systems or deviate from a carefully worded script. These small pilots are being carefully monitored and would only be rolled out if deemed to be successful.

“It is crucial that offenders do not sit idle in their cells. Ensuring prisoners learn new skills through work opportunities makes them more employable in the future – that means a lower risk of reoffending, lower crime and fewer victims.”

The call centre at HMP Oakwood was set up in Spring this year.