Almost 500 British Gas engineers have lost their jobs after they refused to sign new contracts.
The company had given staff until midday on Wednesday to agree to new terms, which will force them to work more hours and cut average pay.
Centrica, which owns British Gas, said around 2% of its staff had chosen not to sign the new contracts and would therefore leave the firm.
However, it declined to give precise numbers saying they could still change.
Andy Prendergast, acting national secretary of GMB union said: "Today has been the largest mass dismissal in living memory and this won't be forgotten.
"Members are upset and angry that after everything, the company has still decided to do this."
'It's not about the money'
Miles Hobson was a British Gas engineer for 38 years before declining to sign his new contract. As a result, on Wednesday, he lost his job.
The Merseyside GMB union representative said his Mum died when he was 30 years old, "so I've had more connection with British Gas than I have had with my Mum."
"Every day I've woken up for 38 years with British Gas on my mind," he told Radio 5 Live. "If you cut me in half there was a flame inside me."
When he finished his last job on Friday he sat crying in his van, and he handed it back in on Monday.
But he would not sign the contract because he felt "bullied".
"We were asked to do more hours for no more pay," he said. "[But] it's not about the money. It's about quality time with [engineers'] families, with their children."
What is the dispute about?
British Gas has changed the contractual terms and conditions for thousands of its workers.
These changes, including to overtime, mean an overall reduction in pay for engineers. British Gas says this amounts to a less than 2% reduction, but the GMB says the cut equates to a 15% reduction in real terms.
Employees were given a deadline of midday on Wednesday to accept those changes by signing new contracts.
Those who refused were told to leave the firm.
How many workers refused to sign?
British Gas said on Wednesday that it expected fewer than 500 engineers "to choose to leave", which represents a bit more than 2% of the workforce.
It said the changes were necessary to protect the company and its 20,000 employees.
Over the past ten years British Gas has lost more than 3 million customers, cut more than 15,000 jobs, and seen profits halve, it said.
The GMB union, however, said that Wednesday had seen "mass sackings" of engineers.
It said that "graveyards of vans" returned by engineers showed that the company doesn't care about "either customers or staff".
What industrial action has been taken?
Wednesday was the 43rd day of strike action taken by thousands of engineers since the dispute started more than six months ago.
The GMB union says this series of strikes has caused a backlog of repairs at 250,000 homes, and that 350,000 planned annual service visits have been cancelled.
Centrica disputes this, saying that many gaps have been plugged by contractors, and that vulnerable people and the elderly have been prioritised for services.
"Unfortunately Covid restrictions coupled with industrial action days have [had an impact on] annual service appointments which we have been rescheduling, and we're in the process of catching up now," a spokesperson said.
Will there be more strikes?
Centrica hopes to draw a line under the dispute, saying the new contracts are "highly competitive", and that it has not cut base pay or final salary pensions.
"Our gas service engineers remain some of the best paid in the sector, earning £40,000 a year," a spokesperson said.
However, GMB union vowed that there would be more industrial action, and said that Centrica chief executive Chris O'Shea was "in dispute with his own staff".
"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," said Justin Bowden.
"But GMB members won't accept the outcome of the bullying... There will be more strikes and action short of strikes."