Hospital workers in Sussex say plans to send patient notes and letters to South Africa or India to be typed could lead to deaths if mistakes are made.
Managers at Brighton and Sussex University Hospitals NHS Trust say the plan could save more than £1m a year as well as clearing a backlog of typing.
"We don't believe it is in patients' interests," said Mark Sargent from public service union Unison.
The trust said a consultation had taken place but no decision had been made.
The trust runs two hospitals, the Princess Royal Hospital in Haywards Heath and the Royal Sussex County Hospital in Brighton.
"The treatment could be incorrect and the outcome could be death for the patient"
The hospitals are among several across Surrey and Sussex whose services are being currently under review.
The programme to "shape the future for health services" in the two counties will involve a public consultation from mid-November and some hospitals could lose A&E or surgical facilities.
But staff have already been consulted about a trial of medical secretarial services, which took place with two companies in South Africa and India.
An internal NHS document, seen by the BBC, said the trial was a success but admits the accuracy of a company in India was "variable".
It gives an example of one mistake where hypertension and hypotension were mixed up.
"If in listening to dictation you hear hypertension and type hypotension - one meaning high blood pressure, the other low - the treatment could be incorrect and the outcome could be death for the patient," said medical secretary Elaine Bass.
If it goes ahead it would reduce local staffing by up to 70 jobs.
"Yes they are going to save money, but we can save money ourselves by doing things in a different manner," said Mr Sargent.
"They have not listened to the staff. If we work together we can make efficiencies for the NHS."
A statement from the trust said: "Brighton and Sussex University Hospitals NHS Trust can confirm that the consultation period with staff about whether to permanently outsource the transcription of some patient letters has ended.
"The Trust is now reviewing the feedback it received from staff before a final decision is made."
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