British gas engineers are set to launch a five-day strike on Thursday in a dispute over pay and terms.
The GMB union said its members were “provoked” to take industrial action and warned of further interruptions if the dispute is not resolved.
UK gas owners Centrica said they had contingency plans to cope with the strike and would give priority to households at risk and emergencies.
Around 4,500 service and repair gas engineers, 600 central heating installers, 540 electrical engineers, 170 specialized business gas engineers and 1,700 smart metering engineers are involved in the strike, the union said.
The only exceptions to the industrial action will be emergencies and problems for households with vulnerable people, according to the union.
The strike follows a 9-1 vote in favor of industrial action by members of the GMB who accused Centrica of cutting wages and terms as part of “firing and hiring” policies.
GMB civil servant Justin Bowden said British Gas’s actions “tarnished” his reputation.
He added: “The use of fire and rehire threats has been condemned across the political spectrum and has caused great anger among this dedicated workforce.”
Mr Bowden said the workers had been “provoked” to take industrial action.
A Centrica spokesman said: “We have done everything with the GMB to avoid industrial action.
“Unfortunately, although we have made great strides with our other unions, the GMB leadership seems intent on disrupting customers on the coldest weekend of the year amid a global health crisis and in the midst of a national lockdown.
“We have strict contingency plans to ensure we continue to serve customers who really need us, and we will prioritize households and emergencies at risk.”
Centrica said it has lost too many customers and jobs in the past few years and has tried to protect jobs.
“If we are to avoid more job losses and, unlike most others in the industry, continue to have a highly skilled team of engineers employed directly by the company, these new terms and conditions are essential.
“A very significant number of striking engineers will have already accepted the new terms,” he said, adding that four out of five employees had accepted new terms that protected basic salaries and pensions.
Hazel Nolan, of the union’s Scotland region, where more than 1,000 UK gas workers are on strike, said: “GMB members are being told they will be fired and then forced to accept new terms – widespread cuts in wages and pensions and walk.
“Take it or leave it. Centrica is transforming what was once a great British industrial institution into a cowboy contractor.”
Strikers will picket during the industrial action.