Talks between the HSE and Caredoc, aimed at avoiding strike action by employees at the GP out-of-hours service, are understood to have broken down.
Both parties recommenced the mediation process but a resolution is not believed to have been reached.
Further engagement between the HSE and Caredoc is likely but it remains uncertain if a withdrawal of labour by workers tomorrow can now be prevented.
Irish Nurses and Midwives Organisation (INMO) and SIPTU members employed at Caredoc previously announced an escalation of planned strike action as part of a pay dispute.
As well as Thursday's planned strike action, there are also plans for 24-hour strikes on 23 and 27 December, a 48-hour strike from 29 December, and a 24-hour strike on 2 January.
Caredoc provides GP out-of-hours services in the south east of the country, Carlow, Kilkenny, Waterford, Wexford and south Tipperary.
However, it also provides these services in south Wicklow, as well as in the north west of the country, Donegal, Sligo, Leitrim and west Cavan.
Its community intervention teams also provide care in the home services in the south east and in Co Wicklow.
The strike action is expected to have a significant impact on these services over the Christmas period, with an estimated loss of up to 10,000 hours of clinical consultations.
There are 150 INMO members and 120 SIPTU members at Caredoc, including triage nurses, drivers for doctors and receptionists.
The unions say they are seeking the implementation of a 2023 Workplace Relations Commission pay agreement for Section 39 organisations, specifically an 8% increase due to members working in Caredoc.
In a statement issued tonight, Caredoc called on the INMO and SIPTU to suspend their planned strike action on "humanitarian grounds".
A spokesperson for the service said: "This 24-hour industrial action coincides with the busiest time of the year for Caredoc, which plays a vital role in maintaining a frontline health service for thousands of patients.
"The current winter flu epidemic, expected to peak over the Christmas period, is proving particularly virulent this year as hospitals and GP services already struggle to provide adequate cover.
"Caredoc therefore calls on the unions involved to call off the 24 hour one-day strike actions scheduled for both 18 December and Monday, 23 December, as such action will only make an already deteriorating situation even worse, and put patients at risk.
"For its part Caredoc, remains committed to intensive engagement with the HSE and to arriving at a fair result that can hopefully restore pay parity between all its staff members and HSE equivalents, as was the case up to 2021."
"Caredoc will continue to make every effort to ensure the minimum possible impact on patients and their families, from the current dispute," the spokesperson added.
Meanwhile, SIPTU has said it is "deeply disappointed" that no resolution was reached at the talks.
The union has also said that it was only when its members served strike notice that management and the HSE "took the issue seriously".
SIPTU has called on the HSE and Caredoc to prevent the strike action from going ahead.
"Our members have voted overwhelmingly in favour of strike action due to their employer, CareDoc, failing to apply the agreed pay increases of 8% that have been due since 2023," SIPTU Organiser Ger McNally said.
"This is a long-running dispute will escalate with strike action on the 18th of December, with further stoppages notified to CareDoc over the Christmas period.
"It was only when our members served notice of industrial action that management and the HSE took the issue seriously and engaged directly on the matter.
"We are extremely disappointed that CareDoc has failed to put forward any proposals to address the claim to date," he added.
"SIPTU members remained available throughout the entire process to engage constructively with management and to consider any meaningful proposals. Regrettably, none have been forthcoming, leaving us with no alternative but to proceed with our planned industrial action.
"We again call on Caredoc and the HSE to ensure that all avenues are fully exhausted in the time remaining before 8.00am on Thursday to find a resolution to our members legitimate claim."
