Hundreds of workers at energy giant EDF are to go on strike in a dispute over pay, warning the action will lead to delays in fixing and installing meters.
Unite said almost 500 of its members in London, the South East, South West and Eastern regions will walk out on May 6 and 8.
The union said the two days of strikes will also hit revenue protection and warned of further industrial action if the row is not resolved.
Unite regional officer Kasab Onay said: "The adverse impact of two days of strike action will be significant in terms of installing meters for customers and in relation to revenue protection.
"Unless the management of this highly profitable company sits down and engages in constructive talks, more industrial action is on the cards."
Almost four out of five workers voted to go on strike in protest at a 2% offer as well as other pay issues.
EDF Energy said in a statement: " We have offered employees working in our metering division a final pay offer of 2% with an additional lump sum of £200.
"The negotiations relate to the 2013 pay deal, which once agreed would be backdated to last April.
"We believe our final offer is fair and competitive. This is part of our commitment to our customers to ensure our costs are controlled and affordable.
"An identical offer was accepted last year by the vast majority of a much larger group of employees in the same part of our business - including those working in our customer service centres.
"We have been in continuous talks with Unite and other unions for more than 12 months to agree the pay increase and we remain open as always to meeting employee representatives."