NHS Barnsley staff will vote on health care being provided by businesses as part of a national shake-up by the Department of Health.
Union officials are worried employees have not been made aware of the significance of the ballot which they say could ultimately result in job cuts and a decline in standards of patient care.
Staff will be issued a 'preference poll' with their July payslips asking them to vote on a new way of providing health care.
The options are to retain an NHS organisation or to go for social enterprise, defined by the government as businesses with social objectives as their main aim.
But Barnsley Unison representative Tony Wright has warned it would be disastrous for the town if health services were broken up.
"We see it as the beginning of the end of a public NHS," he said.
"Groups of staff and outside agencies would be encouraged to bid for services and the people would no longer be the priority, profit would. They would look after business first, the patient second and the worker third."
There was no-one at NHS Barnsley available to comment.
