Hundreds of Northern Ireland Water workers have begun industrial action which they have warned will have a major impact on customers.
The employees have pushed ahead with a work-to-rule protest after negotiations failed to resolve a row over proposed changes to their pension scheme.
NI Water, a state-owned company, has expressed disappointment at the action and has insisted it cannot avoid implementing pension reforms that have already been introduced elsewhere in the public sector.
Union bosses have said that water supply and treatment services are set to be disrupted over the Christmas period.
Union members are adopting a work-to-rule policy which will also see the withdrawal of overtime and on-call repair services.
Members of the Nipsa, Unite and GMB unions voted for the move last week and subsequent talks involving management and Regional Development Minister Danny Kenny failed to avert it.
Nipsa official Ryan McKinney said it was time for the entire Stormont Executive to get involved.
"We have continued to talk but NI Water has made it clear that they cannot postpone the pension reforms without NI Executive approval," he said.
"The DRD Minister has also indicated that he cannot go it alone. If we are to avoid a public health emergency in the coming days then it will take intervention from the NI Executive.
"Once the industrial action starts there will only be a matter of hours before it begins to threaten water supplies, so this time should be used constructively.
"It must be remembered that water workers are not asking for more money - they want to protect their pension and ensure the future security of their families. That's a hope that all of us have at Christmas."
While warning that the action could potentially cause "significant difficulties" for customers, NI Water said it would work to mitigate any potential impact.