Workers at British Gas are to hold a strike ballot over the threat to fire and rehire 20,000 people.
The ballot will open on 1 December and close on 17 December with the first possible strike anytime from New Year's Eve.
Further strikes could be called throughout the winter.
In July British Gas owners Centrica unveiled plans to fire and rehire the entire workforce if they didn’t accept detrimental changes to their pay and terms and conditions.
In August 95 percent of GMB union members said they would be in favour of industrial action in an indicative ballot.
The most the company was prepared to do was delay the fire and rehire threat until January.
Centrica has openly declared its intention to slash pay and conditions to boost profits.
“To win back customers from our competitors and reverse the decline of our business we must have flexibility to give customers what they want, at a price they want and when they need it,” a Centrica spokesperson said.
“Our current terms and conditions are stopping us doing this and modernising the way we work is critical to our success,” the spokesperson added.
Justin Bowden, GMB national secretary, said, “People don’t like threats and bullying and GMB members are no different.
“This is an insulting response to these essential workers - keeping homes across the nation heated in the depths of winter.
“Anger has boiled over as members have now seen the full proposals and how they impact them.
“GMB has responded by giving the go ahead for an official national strike ballot. We will shortly announce the dates for the ballot.
“All the signs are that a national strike in the gas industry is weeks away if British Gas don’t stop their playground bullying tactics which threaten to disrupt energy supplies and services to customers over the Christmas and New Year period.
“GMB’s plans will exempt vulnerable elderly households from the impact of the action.”