Workers at ESB have voted in favour of industrial action over a pension row.
Union leaders are to meet on Friday to consider the next step after almost 90% of Unite members at the electricity supply board backed the action.
Unite official Richie Browne said the result from all union ballots was 87.5% in favour of action and 12.5% against.
"This overwhelming result indicates the strength of feeling over unilateral changes to the staff pension scheme," he said. "Union members are determined to defend their pension rights."
The row centres on a 1.6 billion euro (£1.34bn) deficit in the company pension scheme and the company's decision to pay a dividend of 78m euro (£65m) to the State during the summer.
The company said it is disappointed with the outcome of the ballot and acknowledged the serious impact it could have on consumers.
"ESB has established and agreed procedures to address industrial relations issues," it said. "Earlier this month, in line with those procedures, ESB wrote to the group of unions inviting them to talks with the company."
It said unions indicated they will respond formally to that request following a meeting of the group. "ESB is disappointed at the outcome of a ballot today by the Group of Unions for industrial action," it added.
"We acknowledge the potentially serious impact of this situation for customers and we will continue to work to resolve the issues directly with the group of unions through the established procedures."
The ESB group of unions includes the Energy Services Union (of Ireland) (ESU), Siptu, TEEU, UCATT and Unite.