Northern Ireland Water has warned that planned industrial action by hundreds of workers over a pensions dispute could cause significant difficulties for customers.
The employees are pushing ahead with a work-to-rule protest next week after negotiations failed to resolve the row over proposed changes to their pension scheme.
NI Water, a state-owned company, said it could not 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 now set to be disrupted over the Christmas period.
From Monday, union members at the state-owned company will adopt a work-to-rule policy that will see the withdrawal of overtime and on-call repair services.
Members of unions Nipsa, Unite and GMB voted for the move earlier this week, but the action was only confirmed after talks with NI Water management and Regional Development Minister Danny Kenny failed to reach consensus.
A spokesman for NI Water said: “We fully recognise the rights of employees to consider industrial action but regret it is to be taken at a time which will potentially cause significant difficulties for our customers.
“We can assure the public, however, that the provision of safe, clean drinking water and waste water services remains a priority for the company and its staff who will continue to work to mitigate any potential impact on customers.
“NI Water is not in a position to defer the implementation of changes to its pension arrangements and given its funding model could not expect to remain immune from the public sector pension reform. Indeed the changes made will come into effect some three years after wider public sector reform commenced.”
Nipsa assistant secretary Ryan McKinney, who was involved in the negotiations, said: “The current proposals will mean that many staff will see their contributions increase by £100 per month.
“We heard today from workers who are worried about their ability to keep a roof over their head so they are absolutely determined to resist these changes.
“The unions have set out their position to both the minister and NI Water and unless the pension proposals are taken off the table staff will take action from Monday.
“The trade unions are willing to continue with negotiations right up to and beyond December 22 in order to attempt to find a solution and avoid disruption to communities.”
Mr Kennedy said the proposed industrial action created a “significant risk” of people not having water over Christmas.
“This is completely unacceptable,” he said.
“However, it is not too late to prevent it and I would urge the unions to engage further to find a way forward.
“The key concern for the unions is the reform of the pension schemes in NI Water. The same changes have already taken place in the Civil Service and other public sector organisations. The increase in contributions has actually been delayed for NIW staff.
“In an attempt to avert strike action, I have offered to bring any agreement on pensions between the unions and NIW management to the Executive in January.”
Mr Kennedy added: “While I understand that feelings are running high within the organisation, I stressed that the industrial action has the potential to have widespread impact on families and particularly the elderly and vulnerable, creating a miserable Christmas for many.
“I also recognise that the workforce has worked hard over the last number of years to restore the public’s confidence in the organisation. I am concerned that this could now be undone as a result of industrial action.
“I have urged the unions not to take this course of action and discussions are ongoing with NI Water management to seek to find a solution. NI Water and the department are putting contingency measures in place in the event that no resolution is found.”