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 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."
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."
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