Hospitals are being accused of putting patients' lives at risk by sending confidential and highly technical medical notes abroad for typing.

Private companies are "aggressively" targeting cash-strapped NHS trusts promising huge cost-savings if they "outsource" the work to countries such as India, South Africa and the Philippines.

The contract typists have already made potentially life-threatening mistakes by getting medical dosages wrong and confusing hypertension or high blood pressure with hypotension - low blood pressure, according to the public service union Unison. Dave Prentis, general secretary of the union, said the "very dangerous" practice started in London but was spreading to the rest of the country. Some 30 hospitals, including flagship units such as Cambridge University Hospital and King's College Hospital, were either using such a system or considering it, he said.

Some of the typists in India are doctors and medical students, but it is thought that others without any experience or qualifications are also being used. Most are understood to be paid by the line so that it is in their interests to work quickly, Mr Prentis said.

He pointed out that the work was currently undertaken by experienced medical secretaries who were familiar with the patients and their medical records and would be able to check indistinct audio tapes with the doctor concerned.

Mr Prentis said his union had received assurances from the Department of Health two years ago that there were no plans to "outsource" the service.

"This is not Unison trying to save jobs. This is Unison saying that patients' health and wellbeing is being put at risk," he said. "This is a step too far. It is ridiculous, it doesn't make sense and it causes real problems for patients. If they are going to privatise this, where are they going to stop."

He said patients' records should be up-to-date and accurate. "The consequences of typing errors are too frightening to contemplate. The difference between hypertension and hypotension can be a matter of life or death."