ESB Networks has described planned industrial action by network technicians belonging to the Independent Workers Union as "unlawful" and says it plans to take legal action over the matter.

In a statement this evening, ESB Networks says it is aware that the IWU members are currently engaged in a work to rule, which may escalate to a 24-hour strike from 8 o'clock tomorrow morning.

The company confirmed that there has been "limited impact" to electricity customers as a result of the industrial action to date - but said it was responding as quickly and as safely as possible when faults occur.

It said it was monitoring the situation closely, and ensuring that the necessary contingency plans were in place to minimise the effects on its operations and customers as much as possible.

"This action is unlawful because IWU have balloted their members to progress a claim that has not been made to ESB or the Labour Court. This is a legal matter, and we are dealing with it as such," the company stated.

It said it was "exploring all options", and had notified the IWU that it intends to take legal action in relation to this dispute.

ESB Networks said it remained committed to resolving the dispute, but failed to explain why it had rejected the IWU proposal to refer the dispute to the Workplace Relations Commission conciliation service.

The IWU network technicians previously belonged to Connect, SIPTU or Unite, which are affiliated to the ESB Group of Unions and the Irish Congress of Trade Unions.

However, the IWU is not recognised by the ESB, and is not affiliated to either body.

It claims to represent over 500 of the company's 1500 network technicians who previously belonged to recognised unions - though rival unions estimate that the real figure is closer to 300.

ESB Networks said it would continue to deal with its recognised trade unions through its long-established industrial relations framework, which had supported the ESB and employees in meeting the challenges of a changing energy industry.

It said the unions affiliated to the ESB Group of Unions had confirmed that they were not in dispute with the company and would continue to provide emergency cover.

ESB Networks reminded the public to stay clear of electricity lines and poles, adding that real time updates for faults were available on www.powercheck.ie.