Despite the British Government's pledge not to send confidential data of patients overseas, the National Health Service (NHS) is sending names addresses and medical reports of millions of patients to India for processing.
More than 30 care trusts have begun to send patient details overseas under pressure to cut costs, the Sunday Times reports.
NHS Waltham Forest, one of the primary care trusts that is sending patient data overseas, said it took "all appropriate security measures" to protect the information.
The databases are administered by about 200 workers in Pune, India.
Although companies handling the records in India said security was "paramount", there is a risk of patients being identified if the NHS numbers are matched with anonymous clinical notes carrying NHS numbers.
As part of the process, a set of clinical notes will be based on a consultant's findings during a session with a patient, which he will read into a voice recorder during the appointment.
The recording is then transferred to a computer and sent to India, where it is transcribed.
Workers in India are also producing letters for patients with appointments for cervical smear tests and breast screenings.
British Ministers have been concerned about the confidentiality of patient information since the launch of a 12-billion-pound scheme to computerise health records.
"Given the government's track record of losing data in this country, it is worrying that data are being sent overseas. Every transfer of information adds to the risk of it being lost," John Hemming, the Liberal Democrat MP, said.