British Gas electricity and gas engineers have rejected a contract shake-up which their union says will significantly cut their pay, taking them closer to potential winter strike action.
The GMB said the ballot of 7,500 members saw 86% vote against proposals from parent company Centrica to simplify its structure.
The union claims the plans amount to a pay hit of up to 10% and worse conditions.
It said a positive strike ballot, due to be completed later this month, could result in disruption to engineer services from New Year's Eve at British Gas - the UK's largest household supplier.
But is added that provision of services for vulnerable customers would be put in place.
The GMB said it would be holding a demonstration on Wednesday urging the company to rethink its plans via the slogan 'turkeys don't vote for Christmas', accusing Centrica of presiding over a "fire and re-hire" campaign.
It released the result hours after Centrica revealed that 7,000 front-line office colleagues represented by UNISON had accepted the terms and 4,000 non-unionised staff were in the process of signing new contracts.
The company said: "In June 2020 we announced significant changes to how we operate, in order to create a simpler, leaner business, focused on allowing us to best serve the changing expectations of our customers.
"The changes included delayering our structure, significantly reducing our management population and a proposal to modernise our colleague terms and conditions to reflect customers' changing needs."
It was a response to a tough last financial year for Centrica as a whole that saw chief executive Iain Conn depart.
Headwinds at British Gas included tougher competition, weaker wholesale prices and the government-imposed cap on default household energy tariffs.
The GMB's national secretary, Justin Bowden, said the company's action amounted to a "zero hours contract attached to a bonus scheme" for his members.
"These unacceptable proposals from British Gas would do extensive damage to the work-life balance of thousands of gas and electrical engineers across the country.
"GMB has written to Centrica Chairman Scott Wheway calling for the fire and rehire threat to be permanently removed and for British Gas to return to the negotiating table," he said.
Sky News has contacted Centrica for a response.
