The trade union, IMPACT, is to ballot its 25,000 members on possible non co-operation with the Government's planned health service administrative reforms.
The union says it has had no firm assurances over services, jobs, pay, employment contracts and working conditions.
It claims: "Health service users face further hardship in the new year - and staff and hospitals face chaos - because plans for health service administrative reforms, due to come into force on 1 st January, are still not ready."
It branded the plans as "woefully undercooked".
It said staff were still unclear about their duties and responsibilities, where they would be located, or who their employer would be - despite protracted negotiations. The ballot result is due towards the end of November.
IMPACT National Secretary Mr Kevin Callinan said: "The official line that there will be 'no big bang' cannot disguise the fact that we are totally unprepared to move to a new system, and that patients and services-users face further hardship unless the Government recognises this and steps back from the abyss."
The changes will see existing health boards and other agencies replaced by the new Health Service Executive (HSE), which will include a national hospitals' office and directorates for shared services and primary, community and continuing care.
It is expected that four new regional centres will be lightly staffed. Other functions are to be subsumed by a new Health Information and Quality Authority. Legislation is expected to go before the Dáil this month and complete the parliamentary process before the Christmas recess.