Staff at the ESB have voted overwhelmingly to take industrial action in a row over a €1.6 billion deficit in the company pension scheme.
Workers are opposing the company's decision to pay a dividend of €78m to the State during the summer while the deficit remains outstanding.
The company is also expected to pay a special dividend of €400m to the Exchequer following the sale of certain assets.
87.5% of those balloted backed industrial action, with 12.5% against, with a turnout of 81%.
Four employees have also taken legal action against the company over the issue.
The ESB said it was disappointed with the outcome of the ballot and acknowledged the "potentially serious" impact on customers.
It said unions have been invited to talks but have yet to formally respond.
The ESB position is that following a restructuring of the pension scheme in 2010, under which it contributed a lump sum of €591m, it has no further responsibility for the deficit.
It has also strongly defended the way it has accounted for the pension fund in its accounts.
The ESB Group of Unions will meet later this week to decide whether to accept the invitation to talks, and whether to serve notice of industrial action.