Up to 300 jobs are to be cut at the three main hospitals in Worcestershire, as part of plans to save about £60m.
The administrative and clerical jobs are to go at the Worcestershire Royal, Kidderminster Hospital and the Alexandra in Redditch by 2017.
Worcestershire Acute Hospitals NHS Trust said the cuts would not affect medical staff.
The Unison union claimed the job losses would have a "serious and direct impact on patient care".
The trust said the job losses were not related to the planned £35m reorganisation of health services in Worcestershire which includes the downgrading of the Alexandra Hospital.
It said it needed to make savings of £15m a year over the next four years because of rising costs and it was "inevitable" it would lose some non-clinical posts.
A spokesperson for the trust said: "The reductions will be linked to new ways of working and better use of IT.
"Where posts do need to go we will look to achieve this through natural wastage and voluntary redundancies, with compulsory redundancies being the last resort."
The three hospitals would also use fewer agency staff, the trust said.
Ravi Subramanian, from Unison, said he was "very worried" about the job cuts.
He said: "If those 300 staff are taken away from supporting clinical areas it means clinical staff are going to have to be doing their own admin and clerical work which is going to take them away from the frontline working with patients."