Workers at ESB this evening voted overwhelmingly in favour of strike action over plans to split the company.
The Group of unions are seeking a meeting with management over proposals to separate Eirgrid, which operates the national grid, from the ESB as a generator and supplier to customers. Union officials are due to meet again next week to discuss the vote.
EirGrid, is owned by ESB but managed by an separate State agency, controls electricity distribution through pylons and cables.
A White paper in March this year said the move would help introduce competition into the Irish market, where prices are above the EU average. The paper also proposes restrictions on ESB's power generation.
Jimmy Nolan of the Technical Engineering and Electrical Union (TEEU) said the union supported the "thrust" of Government energy policy but splitting the company would devalue it by €1 billion.
The ESB's competitors say its control of the national grid creates a conflict of interest but European Commission proposals for creating EU-wide competition published in September suggests a break-up would not be necessary.
Mr Nolan said: "We believe such developments could seriously damage the long term commercial viability of the company and, in the process, undermine security of supply for the nation.
"The ESB is also a highly profitable company and contributes more revenue to the Exchequer than all the other commercial semi-state companies combined.
"It makes no business sense and no strategic sense to break it up the company and hobble its capacity to compete effectively in either the national, or international market."