Government plans to share taxpayers' data with private firms were condemned as "borderline insane" by a senior Tory MP.
Under the proposals, HM Revenue and Customs would be allowed to release anonymised information to third parties including companies, researchers and public bodies where there is a public benefit.
The Guardian reported that HMRC documents said "charging options" were being examined by officials, indicating that firms could pay to access the data.
Treasury minister David Gauke is overseeing the plan, with legislation being drawn up by HMRC, the newspaper reported.
In its response to a consultation on the proposals last year, HMRC insisted the principle of "taxpayer confidentiality" would be protected under the reforms.
The document states that the Government has "decided to proceed with the proposal to remove the legal restrictions that currently limit HMRC's ability to share anonymised individual level data for the purpose of research and analysis and deliver public benefits wider than HMRC's own functions".
HMRC accepts that "this must be done only where there are sufficient safeguards in place to protect taxpayer confidentiality".
But Tory former minister David Davis described the proposal as "borderline insane", telling The Guardian: "The officials who drew this up clearly have no idea of the risks to data in an electronic age.
"Our forefathers put these checks and balances in place when the information was kept in cardboard files, and data was therefore difficult to appropriate and misuse.
"It defies logic that we would remove those restraints at a time when data can be collected by the gigabyte, processed in milliseconds and transported around the world almost instantaneously."
Emma Carr, deputy director of Big Brother Watch, said: "There is a massive difference between aggregate data and information that is published at an individual level. The two carry vastly different privacy risks.
"The ongoing claims about anonymous data overlook the serious risks to privacy of individual level data being vulnerable to re-identification.
"Given the huge uproar about similar plans for medical records, you would have hoped HRMC would have learned that trying to sneak plans like this under the radar is not the way to build trust or develop good policy.
"Given those who abuse personal information cannot be sent to jail this is yet another instance where Government should be putting proper protections in place before any more data is shared, rather than just hoping nothing goes wrong. Given the sensitivity of people's financial records that is clearly an inadequate and dangerous approach to take."
An HMRC spokesman said: "No final decisions have been taken, but HMRC remains committed to safeguarding taxpayer confidentiality. "HMRC would only share data where this would generate clear public benefits, and where there are robust safeguards in place.
"Last year's consultation made it very clear that there would be a rigorous accreditation process for anyone wanting access to the data and that any access would take place in a secure environment. Those accessing data would be subject to the same confidentiality provisions as HMRC staff, including a criminal sanction for unlawful disclosure of taxpayer information.
"HMRC will be consulting further and will ask for views on whether to charge to cover the costs of processing and providing anonymised data. This would not be charging for the data itself, purely covering the costs of providing it."
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