The union representing hospital cleaning staff at Noble's says a private contractor will duplicate work already done in-house.
The Department of Health has appointed G4S for a 12-week terminal cleaning trial.
The firm will be called in to deep clean wards after an infection outbreak.
At present the work is carried out by the hospital's housekeeping staff but health managers say that means putting other cleaning duties on hold.
G4S was appointed for the trial without a tendering process - although tenders would be invited for a permanent contract if the trial's a success.
Eric Holmes from Unite says members heard about the change at a meeting last week, after the deal had been finalised:
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