IMPACT has instructed its members not to cover the work of 800 posts axed or left vacant by the Government's health cuts.
The union held a national ballot of clerical and administrative staff and has instructed them to boycott the posts with effect from September 23.
IMPACTS's Bernard Harbour said the health service will feel the difference without the staff covering the gaps.
He said: "All health workers are working longer and harder than they ever have to provide services, so it is not possible to cut that amount of jobs without having an impact on services."