Medical experts have hit out against an NHS computer system which gives researchers access to patient information without their consent.
The Secondary Uses Service (SUS) system gives commercial and academic researchers access to patient information which is 'anonymised'.
But in a letter to the British Medical Journal, Dr Ian Brown of the Oxford Internet Institute said patients were being misled about the level of anonymisation of their data and the likelihood of re-identification.
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"The Department of Health claims that patient data accessible through SUS should be available for use in medical research without consent, because the patient cannot be identified from such data. On the other hand, researchers want to be able to make historical or other linkages with data, and therefore data in SUS are only 'partially pseudonymised'," he said.
Patients are not currently being adequately informed about possible secondary uses of their medical data for medical research are not offered unambiguous and effective opt-outs, he added.
Helen Wilkinson, spokeswoman for the NHS confidentiality campaign group The Big Opt Out, said: "Patients may not be honest with their doctors if they are concerned they may be identified and have something they don't want their family to find out about. Also, if they are giving the wrong information then this could mean their research is not valid."