British Gas engineers will stage their 43rd strike on Wednesday in a long-running dispute over pay and conditions.
GMB said the company is going ahead with sacking engineers who have refused to sign new contracts which the union claims will cut pay by 15%.
The company denies the union’s claims and said it is time to move on.
GMB said further strikes and other forms of industrial action will be held unless the deadlock is broken.
The strikes have led to a huge backlog of services and emergency repairs, said the union.
National officer Justin Bowden said: “That British Gas doesn’t give a toss for either its customers or staff is evidenced by the mass sacking of the engineers that it badly needs to service these customers.
“Whilst there is sadly nothing to stop a company bullying its own staff to sign terms they don’t accept, and sacking those who won’t submit to bullying, GMB members will not accept the outcome of this nine-month campaign of British Gas bullying.
“That is why they are staging their 43rd day of strike action on Wednesday.”
A spokesman for British Gas owner Centrica said: “We are modernising the way we work to give our customers the service they want and protect the future of our company and our 20,000 colleagues.
“The vast majority of our employees have agreed to the new terms, which are fair and very competitive. We’re not changing base salaries or pensions.
“Sadly, the GMB continue to say we have cut pay by 15% and this is simply not true.
“Our gas service engineers remain some of the best-paid in the sector, on £40,000 a year minimum.
“Some of our engineers who don’t currently work a 40-hour week like their other colleagues will be working an additional three hours, and we’re increasing their base pay for the next two years to reflect this.
“After this point we’re still paying for those hours but through a productivity bonus, which means earnings are uncapped.
“It’s now time for all parties to move forward and continue the turnaround of Centrica for our customers’ and our colleagues’ benefit.”