Eight hospitals in London are sending consultants' letters to India to be typed because of a shortage of medical secretaries in the city.
They are using the services of Omnimedical to clear a backlog of letters which need to be transcribed.
But the Association of Medical Secretaries has warned that the use of staff in India could be putting patients' lives at risk.
They say the system means the chances of mistakes is increased.
About 7,000 letters a month are being sent to India from St George's Hospital in Tooting, south-west London, and seven others around the city.
A pool of secretaries in India deals with the letters sent from hospitals to GPs to update them on the treatment of their patients.
After consultants have dictated their letters on to voice recorders, the files are downloaded and sent to Omnimedical.
The company removes all the information which could identify a patient and replaces it with a number.
Rigorous safeguards
When the letter has been typed up by the staff in India, it is returned to Omnimedical, which replaces the patient's personal details before it is returned to secretaries at the hospital for checking.
St George's Hospital said there were rigorous safeguards in place to ensure that letters were accurate.
But Michael Fiennes, of the Association of Medical Secretaries, said he had heard of many examples of mistakes creeping into letters, some so serious that they could lead to patients being given the wrong dose of medication.
In one case he claimed a "below knee amputation" was transcribed as "baloney amputation" and the drug "Lansoprazole" as the holiday resort "Lanzarote".
"Medical secretaries should be properly trained, but they are appallingly badly paid for the work they do and that is why the work is being sent abroad," he said.
"This increases the chances of mistakes creeping in that could put patients' lives at risk."