The Fórsa union has written to the HSE saying that any unilateral instruction to staff to change their hours without full adherence to existing agreements will necessitate an appropriate response.
It follows a memo from the HSE chief Bernard Gloster last week to senior regional managers that an extended working day, working between 8am-8pm and an extended working week, five days over seven days, can be applied to all staff employed or promoted since December 2008.
The HSE is seeking more staffing at weekends when there is increased demand for services.
Earlier this week, the Minister for Health Jennifer Carroll MacNeill said she wanted full use of the public only consultant contract, and more consultants rostered on-call or on site at weekends to help with admissions and discharges and to reduce overcrowding build-up in the days after weekends.
The HSE has asked managers to ensure that maximum effort is applied to expand rosters to an extended day week and said that staff can be given two weeks' notice.
Fórsa has described it as an "aggressive approach" by the HSE and said that the health service must engage with staff through the previously agreed Framework Agreement.
The union said it has consistently expressed its willingness to engage with management on the topic, including discussions held on the sidelines of its 2023 conference.
The union said it was important to emphasise that the failure to engage lies with the HSE, as no proposals have been forthcoming at either the local or national level.
The Irish Medical Organisation has said that hospital staffing levels must increase, if standard weekend working is introduced.
The union said that consultants are not the obstacle to extending routine health services over the weekend, but serious investment in staffing and infrastructure, as well as a fundamental change in how hospital care is delivered, are required to support such a move.
Professor Matthew Sadlier, Chair of the IMO Consultants' Committee, said that almost all consultants are currently on rosters which involve covering unscheduled care on weekends.
He added that as most consultants have signed up to the new contract it is evident that there is an appetite amongst consultants to provide enhanced care over weekends.
Govt expects HSE to abide by Framework Agreement - Minister
Meanwhile, the Minister for Education Helen McEntee said the Government "expects and insists" that the management at the HSE abide by the Framework Agreement reached with trade unions when introducing changes to work practices.
She was responding to the Independent Tipperary TD Seamus Healy in the Dáil who called on the Coalition to instruct Mr Gloster to stop what he termed "aggressive and bully-boy tactics" in the HSE's bid to extend working hours.
He said there had been "no consultation whatsoever" with trade unions, which was a breach of good industrial relations and the framework deal.
Mr Healy warned that if the HSE CEO persists, it would have "serious consequences on [the] public", given that the Fórsa trade union had said the HSE approach would "necessitate an appropriate response" - industrial action.
Ms McEntee said that there were "clear mechanisms" to resolve disputes.
However, she added that the new Minister for Health was identifying how the record resources provided to the health service could be used "in the best way" and then replicated across the system.
Additional reporting Paul Cunningham
