The GMB union said the company is going ahead with sacking engineers who have refused to sign new contracts which it claims will cut pay by 15 per cent.
The company denies the union’s claims and said it is time to move on.
GMB Regional Secretary Justin Bowden said the firm did not “give a toss for either customers or staff”.
He said: “These sacked gas engineers are badly needed by customers to clear the huge backlog of missed planned annual service visits and repairs.
“There is sadly nothing in law to stop corporate bullying by companies of their own staff to sign terms they don’t accept and sacking those who don’t submit to this bullying.
“But GMB members won’t accept the outcome of the bullying. This is why we are staging the 43rd day of strike action today.”
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 per cent 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.”