British Gas-owner Centrica will tell thousands of staff to accept new working conditions, including no extra overtime pay, or risk their jobs.
The firm said if employees don't sign the contract, there will be a fresh wave of layoffs, although it insists that is a "last resort".
Centrica has already outlined 5,000 job cuts as customer numbers tumble.
The firm said it had "been open about the changes" needed to win back customers.
The proposals are all subject to a consultation period with unions, the company stressed.
"Our employees' base pay and pensions will be protected, but simplifying and modernising their terms is essential if we're to become more flexible and price competitive," said Centrica.
"We have over 80 different employee contracts with 7,000 variations of terms, many of which are outdated and stop us delivering for customers."
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Unions and workers said they were concerned about the move and criticised the timing amid lockdown.
"They are using this as an excuse because they know we can't even have discussions and meetings," said one British Gas engineer, who has worked for the firm for more than 15 years and spoke to the BBC on condition of anonymity.
"This really is a divide-and-conquer moment."
The company says it must become more competitive to protect jobs in the long term.
Centrica proposes to fix overtime pay at the same rate as regular hours, according to a presentation seen by the BBC.
Previously, overtime could attract double the hourly rate, depending on a worker's contract.
Engineers who might previously have been asked to work shifts between 08:00 and 20:00 in the busier winter period could be allocated hours any time between 06:00 and 23:00.
Centrica follows British Airways in combining proposed layoffs with new contracts which unions describe as unfavourable. Both companies insist the deal offered is fair.
'Huge slap in the face'
"What is really painful is that when this coronavirus kicked off, we all rose to the challenge," said the engineer.
He and other British Gas workers volunteered to deliver meals for vulnerable people for the Trussell Trust.
This gave him and his colleagues a sense of purpose, he said, together with continuing to repair broken heating systems during lockdown.
"We were going into houses. We were feeling proud, as we were key workers," he said. "It's a huge slap in the face."
Centrica said a so-called Section 188 notice, which employers are obliged to give to workers' representatives if they are considering large-scale layoffs, was a last resort if workers did not agree the new terms.
"We've been open about the changes we need to make to win back customers, grow our company and protect jobs in the long run," the company said in a statement.
The GMB union said it had started talks with the company on planned changes, as Centrica has set a deadline of agreeing a deal with employees before winter.
Assistant general secretary Christina McAnea of the Unison union branded the move "disgraceful behaviour".
"Employees have worked hard throughout the past few months to ensure customers are well-served, despite the pandemic," she said. "This is no way for company directors to repay them."
The company is scrambling to stem the flow of customers from its energy supply business.
Last month, it began trialling a cheaper, digital-only brand under the name British Gas X.
It also already plans job cuts at its head office.
New boss Chris O'Shea said most of the cuts would fall in the UK as the energy giant seeks to slim down its business.
About half of the jobs to go will be among the company's leadership, management and corporate staff. This will include half of the senior leadership team of 40, who will leave by the end of August.
Centrica has about 27,000 employees, with 20,000 of these based in the UK.