British Gas engineers will launch a five-day strike from Thursday in a dispute over pay and conditions.
The GMB union said its members had been “provoked” into taking industrial action, warning of further stoppages if the row is not resolved.
British Gas owners Centrica said it had contingency plans in place to deal with the walkout, and will prioritise vulnerable households and emergencies.
The strike involves around 4,500 service and repair gas engineers, 600 central heating installers, 540 electrical engineers, 170 specialist business gas engineers and 1,700 smart metering engineers, said the union.
The only exceptions to the industrial action will be dealing with emergencies and problems for households with vulnerable people, according to the union.
The strike follows a 9-1 vote in favour of industrial action by members of the GMB, which accused Centrica of planning to cut pay, terms and conditions under moves to “fire and rehire” employees.
GMB national official Justin Bowden said the actions of British Gas had “tarnished” its reputation.
He added: “The use of fire and rehire threats has been condemned across the political spectrum and caused huge anger among this dedicated workforce.”
Mr Bowden said workers had been “provoked” into taking industrial action.
A Centrica spokesman said: “We’ve done everything we can with the GMB to avoid industrial action.
“Whilst we’ve made great progress with our other unions, sadly the GMB leadership seems intent on causing disruption to customers during the coldest weekend of the year, amid a global health crisis and in the middle of a national lockdown.
“We have strong contingency plans in place to ensure we will still be there for customers who really need us, and we’ll prioritise vulnerable households and emergencies.”
Centrica said it had lost too many customers and jobs in recent years, saying it was trying to protect jobs.
“If we are to avoid more job losses and continue, unlike most in the sector, to maintain a highly skilled team of engineers, employed directly by the company, these new terms and conditions are essential.
“A very significant number of engineers striking will have accepted the new terms already,” it said, adding that four out of five of its workforce had accepted new terms, in which base pay and pensions were protected.
Hazel Nolan, of the union’s Scotland region – where more than 1,000 British Gas workers will strike, said: “GMB members are being told they’ll be sacked and then forced to accept new terms and conditions – across the board cuts in wages, pensions and leave.
“Take it or leave it. Centrica are turning a once great British industrial institution into a cowboy contractor.”
Strikers will mount picket lines during the industrial action.